II. NORIAN.-The terrane thus designated by the author in 1870 is composed in great part of those stratiform rocks having a base of anorthic feldspars, to which has been given the name of norite. This terrane, however, includes intercalated strata of gneiss, of quartzite, and of crystalline limestone, all of which resemble closely those of the Upper Laurentian. These norite rocks, which are sometimes called gabbros, are not to be confounded with the very distinct gabbros of the Huronian terrane, nor yet with certain plutonic rocks having with them certain mineralogical resemblances. The facies of the norites serves to distinguish them. III. ARVONIAN.-This terrane is composed in great part of petrosiliceous rocks, which pass into quartziferous porphyries. With them, however, are intercalated certain hornblendic rocks, sericitic schists, quartzites, oxides of iron, and, more rarely, crystalline limestone. This terrane, indicated for the first time as distinct by Dr. Henry Hicks, in Wales, in 1878, and named by him, is regarded by Mr. Charles Hitchcock as constituting in North America the lower portion of the Huronian. IV. HURONIAN.-This name was given by the author in 1855 to a terrane already recognized in North America, where it rests in discordant stratification either upon the Laurentian gneiss or upon the Arvonian petrosilex. It includes, besides quartzose, epidotic, chloritie, and calcareous schists, masses of serpentine, and of lherzolite, together with euphotides, which represent herein the norites of the Norian terrane, often confounded with them under the common name of gabbro. This Huronian terrane is greatly developed in the Alps, where it constitutes the series of the greenstones or pietre verdi. V. MONTALBAN.-The studies of von Hauer in the Eastern Alps, published in 1868, and those of Gerlach on the Western Alps, published in the year following, agree in recognizing in these regions two gneissic terranes-namely, an older or ancient central gneiss, and a younger or recent gneiss; this last, which is petrographically very distinct from the old gneiss, being accompanied by micaceous and hornblendic schists. The studies of Gastaldi, published in 1871, and those of Neri, in 1874, while confirming the results of von Hauer and of Gerlach, furnish us with further details respecting these terranes and their lithological characters. It should here be remarked that all of these observers appear to agree in placing the horizon of the pietre verdi (Huronian) between the ancient gneiss (Laurentian) and the recent gneiss. Before becoming acquainted with the first results of these observers, the writer, from his own studies in North America, was led to precisely similar conclusions, and in 1870 announced the existence of a series of younger gneisses very distinct from the old Laurentian gneisses, and accompanied by crystalline limestones and by micaceous and hornblendic schists. To this younger terrane, on account of its great development in the White Mountains of New Hampshire, he gave in 1871 the name of Montalban. This series appears to be identical with the younger gneiss of the Alps; the so-called Hercynian gneisses and mica schists of Bavaria; the granulites, with dichroitegneiss, mica-schists, and lherzolite of the Erzgebirge in Saxony; and similar rocks in the Scottish Highlands. The Montalban terrane in North America contains not only crystalline limestones, but beds of lherzolite and of serpentine, resembling in this respect the Huronian and the Laurentian. It is in this series, in North America at least, that are found the chief part of the veins or endogenous masses of granite, which carry beryl, tourmaline, and the ores of tin, of uranium, of tantalum, and of niobium. Gastaldi, in an essay published in 1874, declares that "the pietre verdi properly so called" is found between "the ancient porphyroid and fundamental gneiss" and "the recent gneiss, which latter is finer-grained and more quartzose than the other." This younger gneiss he also describes as a gneissic mica-schist, and as a very micaceous gneiss passing into mica-schist, and often hornblendic; the two gneissic series being, according to him, easily distinguished the one from the other. To these two divisions, superior to the ancient gneiss-that is to say, the true pietre verdi and the younger gneiss-Gastaldi adds a third division, still inore recent. This highest division contains considerable masses of strata called by him argillaceous schists, and otherwise lustrous, talcose, micaceous, and sericitic schists. Associated with these are also found quartzites, statuary and cipolin marbles, with dolomite, karstenite, and sometimes hornblendic rocks and serpentines, the presence of which in this division, and also among the recent gneisses, as well as "in the pietre verdi proper," was regarded by Gastaldi as justifying the name of "the pietre verdi zone," often given by him to the whole of this tripie group of crystalline schists, which he recognized as younger than the central gneiss.1 VI. TACONIAN.-This third division, to which Gastaldi did not give a distinctive name, has, as is well known, a very interesting history in Italian geology. A terrane having the same horizon and the same mineralogical characters is found developed on a grand scale in North America, where it includes quartzites, often schistose, and sometimes flexible and elastic, with crystalline limestones yielding both statuary and cipolin marbles. It also contains deposits of magnetite and of hematite, as well as important masses of limonite, which is epigenic in some cases of pyrites, and in others of chalybite, two species which form, by themselves, large masses in the undecayed strata. This same terrane contains, moreover, roofing-slates, as well as lustrous unctuous schists, ordinarily holding damourite, sericite, or pyrophyllite, but including, occasionally, chlorite, steatite, and hornblendic rocks with serpentine and ophicalcite. We also find among these schists, which are met with at several horizons in the terrane, layers which are visibly feldspathic, with others of ill-defined character, which, however, are converted into kaolin by sub-aerial decay. These same schists furnish remarkable crystals of rutile, and also tourmaline, cyanite, staurolite, garnet, and pyroxene. This terrane, which, moreover, appears to be diamond-bearing, was described in 1859 by the late Oscar Lieber, under the name of the Itacolumitic group. Eaton already, in 1832, had pliced the quartzites and the limestones, which form the lower members of this group, in the Primitive division. The argillites in the upper part of the group were regarded as the inferior member of his Transition division, and were, according to him, overlain unconformably by the fossiliferous graywacke (First Graywacke), made the upper member of this same Transition division. In 1842, Ebenezer Emmons inIcluded in what he then named the Taconic system the whole of this crystalline series, to which he added the graywacke; but in 1844 he separated this latter, in which he had meanwhile found a trilobitic fauna, and gave it the name of Upper Taconic ; the inferior and crystalline portions being the Lower Taconic. Many years of study have shown me that this upper division is entirely independent of the Lower Taconic, with which the fossiliferous graywacke series is found in contact only in certain localities, while in many others it rests directly upon more ancient crystalline terranes. Seeing, morever, that the Lower Taconic is found without this graywacke, in a great number of localities, from the Gulf of St. Lawrence as far as Alabama to the south, and as far as Lake Superior to the west; and recognizing also the fact that the Upper Taconic is really a part of the Cambrian (as was avowed by Emmons himself in 1860), the author proposed in 1878 to limit the use of the term Taconic to the crystalline infra-Cambrian series which forms the Lower Taconic of Emmons and the Itacolumitic group of Lieber, and to call it the Taconian terrane. The history of the various attempts made by the partisans of the metamorphic school to establish a more recent origin for the Taconian is a curious one. Various American geologists, adopting for the most part stratigraphical arguments, have successively referred it to the Cambrian, Ordovician, Silurian, Carboniferous, and Triassic horizons. It is, however, to be noted that these same geologists have also maintained the Paleozoic age of the greater part of the other crystalline terranes of North America, comprising the Montalban, the Huronian, the Arvonian, and a part of the Laurentian itself. The want of any conception of the principle of mineralogical development in the history of the crystalline schists, conjoined with the difficulties arising from the stratigraphical complications met with at many points along the eastern border of the great North American Paleozoic basin, has helped to confirm the belief of many American geologists in the hypotheses of the metamorphic and metasomatic schools. 2 § 10. The mineralogical resemblances which exist between the various crystalline terranes above mentioned are easily recognized. This question is discussed at length by the writer (“ Mineral Physiology and Physiography," pp. 457-96) in a study of the geology of the Alps and the Apennines, and of the serpentines of Italy. See als his paper on "Gastaldi and Italian Geology," containing a hitherto unpublished letter from Gastaldi, in the Geological Magazine for December 1887. 2 The reader who wishes to follow this question will find it discussed with much detail in the volume already cited Mineral Physiology and Physiography" (pp. 517-686) under the title of "The Taconic Question in Geology." It is also treated, with some new facts, in the American Naturalist for February, March, and April, 1887, in an article entitled "The Taconic Question Restated." The type of rocks characterized by orthoclase, appearing in the fundamental granite and the granitoid gneisses of the Laurentian, is again found in the quartziferous porphyries of the Arvonian, in the Montalban gneisses, and, though less distinctly, in the feldspathic rocks of the Taconian. The non-magnesian micas, rare in the fundamental granite and the Laurentian gneisses, appear abundantly in the Montalban gneisses and micaschists, as well as in the lustrous schists which are found in the Huronian and the Taconian, and which predominate in the latter. It is further to be remarked that the simple silicates of alumina, such as andalusite, cyanite, fibrolite, and pyrophyllite, as yet unknown in the more ancient terranes, are abundant in the Montalban, and are also found in the Taconian. At the same time, crystalline limestones, oxides of iron, and calcareous and magnesian silicates, are met with in every terrane above the fundamental granite. The chemical and mineralogical differences between these various terranes are more remarkable than the resemblances, a fact which, however, has not prevented some observers from confounding the younger with the older gneisses. Again, the resemblances between the Huronian and Taconian terranes led the late Prof. Kerr, in North Carolina to refer the latter terrane to the Huronian. Moreover, in the vicinity of the Lakes Superior and Huron, where we find alike Laurentian, Norian, Huronian, Montalban, and Taconian, the outcrops of this last were confounded with the Huronian by Murray and by other observers. In 1873, however, the author, distinguishing between the two, gave to the Taconian in this region the provisional name of the Animikie series. It was not until later that he recognized the fact that this series, which is here found in certain localities resting unconformably upon the Huronian, is no other than the Taconian. Emmons, on the contrary, who had long known the existence in this region of what he called the Lower Taconic, believed that the terrane to which the author gave, in 1855, the name of Huronian, was identical with this same Lower Taconic or Taconian. The differences between these two terranes in the basin of Lake Superior, first noted by Logan and later by the author, are clearly brought out by the recent studies of Rominger. Upon all these different terranes, including the Taconian, there rests in discordant stratification in this region a vast series of sandstones and conglomerates, with contemporary basic plutonic rocks, the whole remarkable by the presence of metallic copper. This series, which had been alternately confounded with the Huronian and the Taconian on the one hand, and with the trilobitic sandstones of the Cambrian on the other, was for the first time separated by the author in 1873, under the name of the Keweenaw group, a term changed by him in 1876 to that of the Keweenian terrane. It still remains to be decided whether this series, upon which rest unconformably these same trilobitic sandstones, should form a part of the Cambrian, or should constitute a distinct terrane between the Taconian and the Cambrian. § II. In submitting to his colleagues of the International Geological Congres3 this summary of his conclusions, based on over forty years of study, the author takes the liberty to state that the notions here advanced as to the origin, the chemical and mineralogical history, the subdivision, and the nomenclature of crystalline rocks, are for the most part the generalizations of a single observer. He now offers them as a first attempt at a classification of the indigenous rocks, and at the same time as an exposition of his crenitic hypothesis, and of the mineralogical evolution of the globe, which he conceives to have determined the succession and the chemical nature of the masses which he has named crenitic, as well as those of plutonic masses. He feels at the same time that his work is far from complete, and that to others must now be left the task of correcting and finishing it. As a large part of these results, so far as regards geognostic classification, appeared for the first time in the Reports of the Geological Survey of Canada, the author may be permitted to say, in closing, that the first publications made by that Geolcgical Survey on the crystalline rocks of Canada-that is to say, the reports of progress for the years 1845 and 1846, were prepared by him, and published in 1847, from the notes and the collections made by Logan and by Murray in the two years previous. Moreover, all the statements relating to the mineralogy, the lithology, or the chemical composition of the rocks of Canada, which are found in the official reports from 1847 to 1872, when the author resigned his position as a member of the Geological Survey of Canada, were written by him or under his personal direction. T. STERRY HUNT. SOME QUESTIONS CONNECTED WITH THE PROBLEM PRESENTED BY THE CRYSTALLINE SCHISTS, TOGETHER WITH CONTRIBUTIONS TO THEIR SOLUTION FROM THE PALEOZOIC FORMATIONS.1 an THE question of the "crystalline schists" still presents so many unsolved difficulties, and the views of contemporaneous fellow-workers diverge herein so widely, that attempt at unanimous agreement on the points at issue must at present be regarded as premature. This assuredly does not prevent our taking counsel together, interchanging observations, and endeavouring to gain solid ground, whence a future solution can be aimed at. Each geologist will approach such a consultation in a way differing in accordance with his own experience. I can only contribute experience gained by the study of the metamorphic crystalline schists, belonging to the Paleozoic formations, that have been proved to have resulted from the action of contact or dynamic metamorphism on eruptive or stratified rocks, the latter including the tuffs. The direct application of this experience to all Archæan crystalline schists appears to me premature-i.. rather a thema probandum than probatum. Doubtless there are cases-as, for instance, in the so-called flasergabbros or zobtenites, which, apparently, must be regarded as quite analogous to the alteration of the diabases in the Paleozoic formations. Indeed, the same essential features which Lehmann has described in the development of the Saxon "flasergabbros" have been demonstrated by Teall in the Lizard gabbros, G. H. Williams in the Baltimore gabbros, and Hans H. Reusch in Norway. But Hans H. Reusch also mentions bedded gabbros as well as eruptive flasergabbros, differing thus from Lehmann; while Credner and Roth appear by no means willing to concede all that is contained in Lehmann's book. This fundamental difference must, however, be noticed: Lehmann holds the Archæan schists half for metamorphosed sediments, half for interbedded or injected eruptive rocks; and although I cannot agree with or follow Lehmann in every detail (and, above all, lay more stress upon the altered tuffs), still on the whole I can but support him in this view. Roth, on the other hand, holds all the Archæan crystalline schists-limestones, quartzite, gneiss, mica schist, amphibolite, &c.-for schistose, plutonic (only in form not eruptive) rocks (Erstarrungski uste); finally, Credner holds the majority of the crystalline schists, including granite-gneiss and flaser-gabbro, for the normal stratified sediments of a primaval ocean, their crystalline nature being essentially not due to metamorphism. I have dwelt thus at length on this point in order to demonstrate that there exist numerous controversies even on those questions that admit of solution by reason of the most undoubted pseudomorphic changes (hornblende after diallage, hypersthene, augite; zoisite, epidote, actinolite, quartz, albite after lime-soda feldspar), and by reason of the presence of the original eruptive structure. My stand-point is identical with that expressed by Carl Friedrich Naumann in the following words: My task above all else is to study the metamorphism, with respect both to substance and to structure, of the fossiliferous sediments and the eruptive rocks, together with the tuffs intercalated therein. Much has already been done, especially with respect to contact-metamorphism, which is more sharply defined than regional or dynamic metamorphism. There remains, however, much to answer, especially as the primary structures of original schistose eruptive rocks and the structure and substance of certain very common sedimentary rocks (as, for instance, the greywackes, the socalled greywacke schists, or the majority of the tuffs) are still too little known to afford a firm basis for the study of metamorphic processes. Still the detailed solution of the following question would be of no little value for the study of the Archæan schists:-(1) What material agreement or difference exists between the I Einige Fragen zur Lösung des Problems der krystallinischen Schiefer, nebst Beiträgen zu deren Beantwortung aus dem Paläoz icum," von Prof. Dr. K. A. Lossen. Etudes sur les Schistes cristallins," 1888. Published by the International Geological Congress in London, 1888. (Translated from the German by Dr. F. H. Hatch.) 2 Giving a s mewhat wide meaning to the word "gabbro"; he now says, "dioritic rock," "altered gabbro and diabase." In the Hartz the interesting gabbro-district of Hartzburg presents, among numerous other varieties, some which show layers alternately richer in plagioclase and diallage (bronzite) or present flaser structure with biotite, and possess thus a bedded-like but 1ot a true ledded parallel structure. These rocks are true eruptive gabbros. results of metamorphism due to the contact of granite with fossiliferous sediments and the eruptive rocks intercalated therein, on the one hand, and the Archæan schists on the other? For such a comparison useful data are furnished by the Hartz. These mountains, consisting of fossiliferous sediments and the most diversified eruptive rocks, already plicated at the Coal-measure period, represent a fairly average section of the earth's crust, i.e. although there is no axis of crystalline schists, the strata, together with diabases, keratophyres, and the accompanying tuffs, are considerably depressed between highly elevated plutonic rocks (granite, gabbro, &c.). minerals which compose granites, quartz-diorites, gabbros, diabases, in short holo- and phanero-crystalline eruptive rocks, and that of the secondary minerals produced in the contactmetamorphism of these eruptive rocks? This question must be the more carefully answered, as, in spite of the rich material so excellently collected and cleverly arranged for the use of science by H. Rosenbusch, the order of crystallization of the eruptive rocks is not yet firmly estab lished. A certain degree of regularity is undeniable; but, of the one hand, the chemical law is, as Lagorio has demonstrated, more intricate than that formulated by Rosenbusch; and on the other, the order varies quite unaccountably with alterations in the physical conditions of consolidation (compare granite and pegmatite). (3) Is the ophitic (diabase-) structure under all circumstances the structure of an eruptive rock, or are there undoubted sedimentary rocks possessing a similar structure? (4) It has been proved that graphic granite, as micro- and macropegmatite, forms an integral part of true eruptive rocks, especially of granite and its porphyritic modification. Since graphic granite is very common among the gneisses, the question arises whether it is to be regarded as a true eruptive rock, or whether such occurrences can be proved to have been produced by thermal action, or even lateral secretion, in the sense of a partial solution of the neighbouring rocks. The contact-zones around the gabbro and granite present the following authigenic minerals: quartz, orthoclase, albite, plagioclase, biotite, muscovite, hornblende, actinolite, augite, bronzite, chlorite, epidote, garnet, vesuvian, tourmaline, axinite, wollastonite, cordierite, sphene, spinel, andalusite, rutile, magnetite, hematite, titaniferous iron ore, magnetic pyrites (pyrrothine), and other sulphur ores, calcite, fluorite, apatite; and continued investigations will easily add others to the list, as, for instance, anatase, zoisite, lithionite, lepidolite, corundum, sillimanite, cyanite, graphite-indeed, the four last-mentioned minerals have already been detected in certain mineral aggregations in post-granitic dykes of the Hartz, that probably are to be referred to metamorphic influence. But not only do these minerals show great resemblance to those which are most frequently present in Archean crystalline schists; their combination to definite mineral aggregates and rocks also makes the analogy even more complete. In the normal gneisses, which are derived, with great diversity of structure, from the culm-greywackes and the greywacke schists of the Oberhartz, in contact with granite and gabbro, are intercalated cordierite- and garnet-gneisses and augite (or bronzite) bearing gneisses, which are produced by the alteration of schistose and calcareous sediments. Saccharoidal quartzites are clearly produced by the recrystallization of Carboniferous or Devonian lydites (Kieselschiefer); and it is very difficult to distinguish these from rocks produced by the contact-metamorphism of nearly pure quartz-sandstone (Quarzitsandsteine). Horn-solidation from a magmatic condition, so long as contrary proofs stones (cornéenne), which contain garnet, amphibole, augite (or bronzite), schorl, andalusite, apatite, as well as orthoclase and plagioclase, are found replacing mica-schists and phyllites. The thin limestone-seams in the Lower Devonian (Hercynian), Upper Devonian, and the Culm-measures, are partly metamorphosed to compact or phanero-crystalline "lime-silicate-hornstones," containing garnet or other allied silicates-vesuvian, epidote, malacolite, cordierite, amphibole, sphene, &c., in places also fluorite or axinite, and corresponding to the garnet-rocks, epidote-rocks, pyroxenites, ecklogites, &c., of the Archæan formation. In part, however, they have undergone marmorosis, while being impregnated with garnet or other silicates and locally with ores; even anthraconite is not altogether absent from these marbles. Amphibolites are in part also derived from calcareous sediments; those, however, that contain felspar (plagioclase) in any essential quantity can be demonstrated to result from the contact-metamorphism of pre-granitic, Devonian, and Carboniferous diabases that have been plicated and metamorphosed in common with the strata. Further, there are, in the granite and gabbro contact-zones, alteration products of the diabase that are rich in biotite; and other pre-granitic eruptive masses, such as the augite-keratophyres and the augite-orthophyres, show a great abundance of biotite, which is associated with a recrystallization of the orthoclase and of a part of the augite. This biotite is certainly developed at the expense of chlorite derived from augite or primary hornblende. Schistose rocks with more abundant biotite, that are locally present among the more dominant massive rocks, bear the strongest resemblance to garnetiferous mica-schists. In the porphyroids of the Hartz, which occur both within and without the contact-zones, we mainly find sericitic muscovite; beyond the contact zone it occurs in such abundance as to produce very schistose sericite rocks, which, on the other hand, are here also derived directly from the porphyritic massive rocks. These porphyroids I regard, from my present stand-point, as the metamorphosed pre-granitic tuffs of quartz-keratophyres and quartz-porphyries. To these tuffs are perhaps related certain hornstones, very rich in orthoclase, which occur in the granite contact-zone with Devonian and Carboniferous siliceous schists (equivalents of Adinole?). Other questions are: (2) What differences exist in the order of crystallization of the Even if it be admitted that all minerals can be produced, by a suitable variation of the conditions, either by consolidation, by separation from aqueous solutions, or by sublimation, still it does not follow, to my mind, that all the structures that combine minerals to regular aggregates, can be produced in like manner in these three modes of formation. It seems to me that such structures-as, for instance, the ophitic (diabasic) or the pegmatitic (to say nothing of the structures which are developed in rocks containing glass or other base)-that have been demonstrated to be characteristic of rocks of undoubted eruptive origin, must rather be regarded as indicating an origin by conare not forthcoming. No one, to my knowledge, has ever maintained that the ophitic or diabasic structure can be of sedimentary origin; but gabbros have been claimed-wrongly, as I believe— as sediments, in spite of the close relation of their structure to that of the diabases. As regards graphic granite (or macro-pegmatite), the case is somewhat different. The frequent occurrence of such masses in gneiss has created the notion that they are integral components of the sedimentary gneisses. And this view is maintained, although a considerable portion of these pegmatitic masses can be clearly seen filling vein-like cavities, while another part make up lenticular patches that follow, more or less, the dip and strike of the schists. The occurrence of simple aggregates of quartz and feldspar, that are of thermal origin, mu-t, then, in accordance with one's experience of regional and contact-metamorphism, be unconditionally conceded; while the absence of such aggregates in the greywackes appears to me to absolutely disprove a development by lateral secretions. It is therefore not inconceivable that the pegmatitic aggregates represent, so to speak, the quintessence of the gneiss, exuded into primary cracks. At the same time, great caution is to be recommended; for, since the introduction of the microscope, micropegmatite has, little by little, been recognized as an essential constituent of numerous acid and basic (with SiO, per cent. as low as 48) rocks. The veins of graphic granite in the Hartzburg gabbro have been held by some for segregation-veins. They are, however, demonstrably apophyses of the eruptive granite; indeed, the principal mass of granite in the Brocken massif is, in the main, micropegmatitic. The banded structure, with bilateral symmetry, of many pegmatites, which has been compared to that of many mineral veins, is no proof of their non-eruptive nature. The augites, felspars and other minerals of lavas present banded structures with variable chemical composition: banded structure with a chemical composition varying from that of diabase to graniteporphyry, is shown by compound eruptive dykes, as has lately been well shown by Bücking, in the Thüringerwald ("Jahrb. d. kgl. preuss. Geol. Landesanst. f. 1887," p. 110, et seq.). Even the drusy character and the richness in minerals presented by the central portion of many pegmatite-dykes finds its analogy in the external shells of true eruptive granites, which may, however, be complicated by the influence of thermal actions, accompanying, or subsequent to, eruption. Giant spherulites, of a decimetre diameter, composed of macropegmatite, enveloping a porphyritic Carlsbad twin of potash-feldspar (orthoclase or microcline), that occur in the granite of the Riesengebirge, repeat, on the large scale, the microscopic characters of the micropegmatite of certain quartz- and granite porphyries (the granophyre of Rosenbusch). All these phenomena compel the assumption that at least a part of the pegmatites are of indubitably eruptive origin, and arouse in us the question whether this structure is not to be brought into connection with the origin of the gneisses. (5) What are the differences between the primary structures (due to consolidation) of the plutonic and volcanic rocks and the structures of (a) the crystalline sediments, (b) the metamorphic rocks in contact with granite, (c) the crystalline schists? (6) What reliable characters have we, to distinguish crystalline grains developed in situ from clastic grains, in cases where they occur, side by side, in one and the same rock? The answer to this question has already frequently been attempted, among others in the most praiseworthy manner by A. Wichmann. It requires, however, a fresh solution based on the latest experiences. The safest test of the authigenic, non-clastic nature of a grain is doubtless the presence in it of enclosures of minerals that are also present in the rock as authigenic constituents. External form and internal molecular relations, in consequence of pressure-phenomena, can, however, be very misleading. Hard minerals, especially, occur in clastic sand in very sharp crystals (quariz, tourmaline, zircon, &c.). (7) Are the views of those authors justifiable, who conceive certain gneisses or porphyroid crystalline schists to have been produced by the injection of a granitic magma, in discontinuo, between the schists (Schiefer)? (8) If the views expressed in the preceding question are justifiable, how are the gneisses and porphyroids, produced by the addition of granite in discontinuo to slaty sediments, to be distinguished (a) from true eruptive granite or its porphyritic modification, both having, under the influence of pressure, undergone a "phyllitic" modification; (b) from slaty sediments in which aggregates or crystals of silicates have been deposited from water (quartz and feldspar)? (9) What differences can be established in mineral composition and structure between a true eruptive granite and an indubitably stratified (not simply jointed or cleaved) so-called "Lagergranit' or granite-gneiss? An amalgamation of eruptive granite with the mineral aggregates of the rocks in contact has, according to my experience taken place in some cases; but I have not yet observed an undoubted discontinuity in such granitic material. It is much to be desired that the French geologists (for instance, Michel-Lévy and Charles Barrois), who defend the views formulated in Questions 7 and 8, would enlighten us by good drawings of macro- or microscopic sections, as to how far in this difficult question an incontestable separation of injected ruptive granite from metamorphic gneiss is possible. This would, without doubt, facilitate the solution of Question 9. Unanimity on this point will scarcely be obtained without a careful structural diagnosis, which, of course, must be supported by serviceable material, self-collected in the field. (10) Are there any absolute material and structural differences between metamorphic rocks of the granite contact-zone (hornstones, cornéenne, &c., cp. Question 1) and rocks affected by regional or dynamic (Dislocations-) metamorphism? or are such differences only relative, and what are they? The exact solution of this question requires, above all, the assumption that only such occurrences shall be submitted to consideration that are unmistakably connected with visible eruptive rocks. It should also not be forgotten that rocks which have originally undergone contact-metamorphism have, in some cases, subsequently lost their peculiar characteristics in consequence of the influence of regional metamorphism. With this qualification I am personally inclined to concede only a relative and not absolute differences. I am guided in this, not only by my experience in the Hartz, which has made me acquainted with the remarkable variation of the metamorphic rocks in contact with granite, according as they occur just outside the contactzone or in its outer, middle, or inner division; or again according as they belong to the unpenetrated but eroded mantle of the eruptive cores, or to masses, of greater or smaller extent, that have sunk deep in between the eruptive masses and have been covered up by them. The rocks occurring thus differently vary between a phyllitic clay-slate and gneiss, while the main mass of the slate- and grauwacke-hornstones present little resemblance to the crystalline schists. In the classic region of the Erzgebirge, however, there occur, according to the careful investigation of our Saxon colleagues, compact hornstone-like or even conglomeratic greywacke gneisses (the mica-trap of older writers) that present this analogy in a complete degree. The same analogy is presented by Gosselet's Lower Devonian "cornéite" (to be distinguished from cornéenne, the product of contact-metamorphism) from the regionally metamorphic Ardennes of Belgium. Again, the Lower Devonian fossiliferou sediments of the Ardennes, containing garnets, hornblende, and graphite, that are so well known through A. Renard's admirable descriptions and drawings, remind one of hornstone, although no contact with eruptive rocks has been observed affecting either them or the Cambrian garnetiferous "Wetzschiefer" of Vielsalme. The association of such hornstone-like rocks with those of the usual phyllitic type of regional metamorphism recalls the occurrence of lime silicate-hornstones in the outermost zone (beyond the zone of the "Knotenschiefer" around the granite of the Rammberg. Whatever explanation of these phenomena may be given-Gosselet is decidedly in favour of dynamic metamorphism as opposed to a latent contact-metamorphismat least this is evident, that important contributions to the question, here formulated, can be furnished by the Ardennes. ness. ON THE CLASSIFICATION OF THE THE most important constituent of the earth's crust-the crystalline schists-has remained, with respect to their field-relations and their origin, the most shrouded in darkThe difficulties that bar the way are quite exceptional. We have frequently to deal with rocks that have undergone subsequent alteration, without being able to determine their original constitution, and without being able to explain the nature of the change. We have, as it were, to deal with an equation with two unknowns-we cannot solve it. At the present time we meet with a number of attempts to classify the crystalline schists, mainly according to petrological characters, in stratigraphical groups. I regard these attempts as premature, for this reason: microscopists are unfortunately very behindhand in the exact investigation of the crystalline schists, and of the half-clastic, half-crystalline sediments. The purpose of these lines is to direct attention to another difficulty which has not yet received sufficient consideration, but which bars the way to every attempt of that kind-namely, the mechanical metamorphism during mountain-formation. That, by the plication of the Alps, the constitution of the rocks has been completely changed, is most directly proved by an examination of the sedimentary rocks; because the latter can be also studied in an unaltered condition in adjacent localities. The commonest changes met with here in connection with folding are: Deformation of fossils, pebbles, or crystals (compression in one direction, extension in another) Cleavage (Transversalschieferung). Cleavage with linear extension. Puckering. Internal formation of breccias and cementing of the same by secretions. Internal formation of innumerable slickensides, so as to change the whole structure. Scaly structure, produced by the compression of oolitic structure. Alteration of hematite and limonite into magnetite, in connection with cleavage. Marmorosis of the limestones. Formation of confusedly "kneaded" structures (Knetstructuren). Development of new minerals (garnet, staurolite, mica) in places that have undergone crushing. Now, sedimentary rocks, metamorphosed in the above way, are frequently found in extremely narrow synclinal zones, nipped in between rocks belonging to the crystalline schists. The I "Zur Klassification der krystallinischen Schiefer," von Prof. Dr. Albert Heim. "Etudes ur les Schistes Cristallins." Published by the International Geological Congress in London, 1888. (Translated from the German by Dr. F. H. Hatch.) Massive crystalline, schistose, and even gneissose. They can scarcely be Alpine zones, which consist mainly of crystalline schists, are Now it is in the crystalline schists that the plications of the earth's crust are most potently developed. The isoclinal and fan-shaped folds, the wedging and "kneading together at the contact with the sediments-in short, all these high forms of dislocation, which are the earliest to modify the inner structure of rocks, are to be found in the crystalline zones of the Alps. They are most highly developed in the northern series of the central massifs (Mont Blanc, Aiguille Range, Finsteraar-massif, Gotthard-massif, Silvretta-massif, &c.). At first sight it appears as if the crystalline schists and the true sediments, in the Alps, were separated by a constant unconformity; but frequently even recent sediments are found folded in, parallel with the crystalline schists. Again the sediments often take the position of a central massif; indeed, it seems as if a great part of several of the central massifs consisted of Paleozoic sediments. On the other hand, in the southern central massifs of the Central Alps, we see the crystalline schists lying in all respects like the sediments. Those who have worked in these parts of the Alps will have remarked how often the mechanical crushing undergone by the rocks obliterates the limits of stratigraphical and petrographical characters, and how many rocks have become confused thereby in their development (Ausbildungsweise). Such changes can sometimes be directly proved to be the result of local crushing; sometimes, however, they are regional, and then passages into the unaltered rock are difficult to trace. All degrees of change by earth-movements are to be found, from a slight alteration of the structure up to complete metamorphism. In hundreds of places one does not know whether one has to deal with the residual traces of original bedding or with a cleavage (Transversalschieferung, Quetschungsschieferung) that has completely obliterated the original structures. In many cases it is impossible to distinguish between a schistose structure (Schieferung), superinduced by earth-movements, and one that is original. Schistose structures which cross another are by no means rare. Whether the more pronounced or the less definite one is then the original is often not to be decided. Even an exact microscopical examination will often not suffice to distinguish between structures resulting from crushing and lateral deformation, and the fluxion-structure of an eruptive rock. It is certain that a structural modification by earth-movements has everywhere taken place where linear extension abounds. The latter is never original. In such crystalline schists with linear-parallel structure there are often elongated, ragged mica-scales. The linear extension can go as far as the development of rod-like separation (stenglige Absonderung). one Are there any rocks left in the central massifs of the Alps which have undergone no change in structure during the orogenetic processes? The metamorphism can penetrate still deeper. Enormous zones, for instance, in the interior of the Finsteraarmassif, that were formerly held to be true crystalline schists, prove to be originally clastic rocks of the Carboniferous period that have been squeezed into schists, and pervaded by secondary mica. Conglomeratic rocks of the Verrucano group, and clay-slates, nipped into the central massif, have become We arrive at this conclusion:-The constitution of the crystalline schists in the Alps has been much changed by the orogenetic process (dynamic metamorphism). Original material and material mechanically produced at a later period, are often not to be separated from one another. Besides these, the Alps present other difficulties that stand in the way of the recognition of a stratigraphical grouping of the The field-relations are frequently so intricate, crystalline schists. that often it is very difficult to decide what originally lay under and what above; and whether the enormous thickness, for instance, of many gneiss-complexes, is real, or merely produced by repetitions of the folding, the folds being concealed by cleavage. It follows that, if, on the basis of petrographical relations, a general stratigraphy of the crystalline schists is to be attempted, this must never take place as the result of observations made in plicated regions of the earth's crust: districts must rather be chosen which are not influenced by disturbances of the Alpine character. In the question of the stratigraphy of true crystalline schists, the Alpine geologist is not in the position to furnish material of essential value; he must rather wait for the results of the workers in other regions, in order to be able to apply them to his own district. The dislocations of fractured regions have, in the main, left unaltered the constitution of the rocks. There, then, the crystalline schists can be studied in their unaltered condition. There also they lie in flatter and more regular bedding; and a stratigraphical sequence is sooner to be found than in the Alps. I ON THE ORIGIN OF THE PRIMITIVE N this paper the author briefly summarizes the ideas prevailing (1) Stratigraphical Considerations.-The primitive crystalline rocks form the fundamental floor upon which lie the earlier detrital deposits, their schistosity being often parallel to the stratification of the latter. Although composed mainly of acid gneisses, the primitive rocks present countless variations in chemical and mineralogical composition; they include very basic representatives, such as the amphibolites, pyroxenites, peridotites, cipolines, and dolo| mites, &c. These intercalations are always parallel to the schistosity they form elongated lenticular patches, of which the greater axis is in the direction of the general banding. At the same time, their relative homogeneity in composition is shown by comparison of sequences established, not only in Europe, but also in the United States and the rest of the world. Acid gneisses predominate at the base; then come frequent intercalations of mica-schists and leptynites, with which are "Sur l'Origine des Terrains Cristallins Primitifs," by M. A. MichelLévy, Bull. Soc. Géol. France. ze série, t. xvi. p. 102, 1888. Published by the International Geological Congress in London, 1888. (Abstracted from the French by Dr. F. H. Hatch.) |