together, while their S. poles form the two outer branches of the W. The middle bar fits the center of the coil, while the outer ends nearly touch it on the outside. The number of lines of force perpendicular to the direction of its motion which are cut by the coil is much increased by this disposition of the poles of the magnet.

Bibliography of North American Invertebrate Paleontology, by C. A. White, M.D., and H. Alleyne Nicholson, M.D., etc. 132 pp. 8vo. Washington, 1878. Department of the Interior U. S. Geol. Survey of the Territories; Miscellaneous Publications, No. 10.

Annual Report of the Chief Signal Officer to the Secretary of War, for the year 1877. 570 pp. 8vo, with twenty charts. Washington, 1877.

Annual Report upon the Survey of the Northern and Northwestern Lakes and the Mississippi River in charge of Gen. C. B. Comstock and Capt. H. M. Adams. Appendix LL of the Annual Report of the Chief of Engineers for 1877. Washington, 1877.

Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, Mass. Vol. iv. The terrestrial air-breathing Mollusks of the United States and the adjacent Territories of North America, described and illustrated, by W. G. Binney. Vol. v. Text, 439 pp. 8vo; vol. v. with 74 plates for vol. iii, and 16 plates of vol. v.

Observations and orbits of the Satellites of Mars, with data for Ephemerides in 1879, by Asaph Hall, Prof. Math. U. S. Navy. Rear-Admiral John Rodgers, U S. Navy, Superintendent of the Naval Observatory. 46 pp. 4to. Washington,

1878.

Anales de la Oficina Meteorologica Argentina, por su Director Benjamin A. Gould; Tomo I, Clima de Buenos Aires. 522 pp. 4to, with 17 plates. Buenos Aires, 1878.

Metals and their chief industrial applications; being with some considerable additions the substance of a course of lectures delivered at the Royal Institution of Great Britain in 1877, by Charles R. Alder Wright, D. Sc., etc. 191 pp. 12mo. London, 1878. (Macmillan & Co.)

The Ancient Life-history of the Earth; a comprehensive outline of the principles and leading facts of paleontological science, by H. Alleyne Nicholson, M.D., Ph.D., etc. 407 pp. 8vo. New York, 1878. (D. Appleton & Co.)

OBITUARY.

Rev. W. B. CLARKE, a geologist of eminence in Australia, as well as a clergyman of the Church of England, died on the 16th of June last at St. Leonards (near Sydney), New South Wales, at the age of eighty-five. Mr. Clarke was an enthusiastic worker in geology. His labors in Australia were continued for more than forty years, and resulted in many important discoveries and great progress to Australian geology. The first announcement of gold in Australia was claimed by Mr. Clarke; and from that time, in 1851, Australia began its career as a gold-producing continent. Mr. Clarke was an active member of the Royal Academy of New South Wales, and for several years its Vice-President. His scientific publications are papers read before the Geological Society of London; anniversary addresses as Vice-President of the Royal Society; and various pamphlets on geological discoveries in Australia and Australasia, with one on the Causes and Phenomena of Earthquakes especially in relation to shocks felt in New South Wales and in other Australasian Provinces. He was a man of great excellence and of earnestness in his parish work as well as in his field explorations.

THE

AMERICAN

JOURNAL OF SCIENCE AND ARTS.

[THIRD SERIES.]

ART. XXXVIII.-- On some points in Lithology; by JAMES D. DANA.

I. ON SOME OF THE CHARACTERS EMPLOYED IN DISTINGUISHING DIFFERENT KINDS OF ROCKS.

LITHOLOGY is a department of Geology, rocks being the material in and through which geological problems are presented for study. The true aim of the science of lithology is to describe the kinds of rocks mineralogically and chemically, and to note down their distinctions, in such a manner as shall best contribute to the objects of geology; and these latter objects include, as regards rocks, the origin of the minerals, and mineral associations, constituting or occurring in rocks: the origin of the rock masses and their relations to other geological phenomena; and the origin of all changes or transformations that have taken place in rocks in the course of the earth's physical development. Geology, chemistry and mineralogy have each to be considered in determining the proper distinctions between the kinds of rocks. Should lithology make much of mere difference in texture, or in ingredients that are present only in minute proportion, geology might rightly say that, for such a purpose, these points are of small importance compared with the nature or composition of the mass.

The defining of rocks is attended with special difficulties on account of their mutual transitions. From granite down they are, with very few exceptions, mixtures of minerals, as much so as the mud of a mud bank. They graduate into one another by indefinite blendings, as the mud of one mud bank graduates into the mud of others around it. In fact a large part of the AM. JOUR. SCI.-THIRD SERIES, VOL. XVI, No. 95.-Nov., 1878.

crystalline rocks were once actual mud beds or sand beds; and even part of the eruptive rocks may have been so in their earlier history. Strongly drawn limits no where exist. Rocks are hence of different kinds, not of different species; and only those mixtures are to be regarded as distinct kinds of rocks which have a sufficiently wide distribution to make a distinct name important to the geologist. Other kinds have to be classed as varieties, if worthy of that degree of recognition."

แ

In the following pages I propose to consider the value of some of the distinctive characters which are generally accepted at the present time in defining certain kinds of rocks.

1. "Older" and "Younger."-The distinctions "older" and younger," often applied to a number of kinds of eruptive rocks, seem to imply that the earth has generated different kinds of rocks as it has grown old. The terms have reference, however, to only one epoch of abrupt change that between the Cretaceous and Tertiary, "older" signifying pre-Tertiary and "younger," Tertiary or later in date. It is of eminent importance to geology to know definitely whether this epoch was one of great change in the earth's ejections, and an epoch so marked that the rocks on one side of the time-boundary are deserving generally of different names from those of the other; for thus lithology, judging from some recent works, as well as older, has seemingly decided.

Some examples of the "older" kinds are dioryte, diabase, and a large part of felsyte; and some of the "younger" are propylyte, doleryte or basalt, and trachyte. The value of the distinction may be learned from a comparison of the rocks of one of these series with the rocks of the other.

First as to diabase and doleryte. Typical diabase consists, according to the descriptions, of labradorite and augite, with some magnetite or titanic iron: and so does doleryte. Diabase. to a large extent, is a crystalline-granular rock; so is doleryte. Diabase was formerly supposed to be peculiar in containing chlorite, but it is now proved, as asserted by Rosenbusch, that chlorite is not an essential characteristic, so that diabase may be chloritic or not; and the same is true of doleryte. Old diabase was described as differing from the younger rock doleryte in containing no glassy portions or grains among the crystalline grains; but this also is set aside by later observations, and Rosenbusch accordingly divides diabase into (1) massive granular diabase, (2) diabase-prophyrite, and (3) glass-bearing diabase; and corresponding subdivisions are as good for doleryte. Thus in chemical composition, in mineral composition, in texture, in the presence or absence of chlorite, in the presence or absence of glassy portions, the two rocks are identical. Analyses of "diabases" from the Archæan to the Tertiary, and of

dolerytes" of subsequent time, have shown that material of essentially the same chemical composition, has been ejected in all geological ages, as has been well urged by Allport and others. The analyses might be cited; but this is not necessary, since in mineral composition typical diabase and doleryte are admitted to be identical.

The fact as regards these two rocks, then, give no foundation for the idea of such a transition epoch in rock-making at the close of the Cretaceous period. And if not, it is bad for geology to have such epithets as "younger" and "older" treated with so great distinction.

Again the difference between dioryte ("older") and propylyte ("younger") is not in the chemical or mineral composition of the rocks; and hence, whatever difference there be is only in texture and is, therefore, of little geological value.

Again, felsyte and trachyte are rocks of one and the same chemical and mineral constitution. Ordinary felsyte consists of orthoclase, or orthoclase and oligoclase, with sometimes disseminated hornblende or quartz; and the same is precisely the constitution of kinds of trachyte. They differ in aspect, and feel differently under the fingers; and still some varieties of felsyte differ from ordinary trachyte only in having the disseminated orthoclase crystals not translucent, a difference of very small value mineralogically and not less so geologically. The rock of certain felsitic dikes in Canada and Vermont, Paleozoic in age, is called trachyte by T. Sterry Hunt, in the Canada Geological Report, because of the essential identity with that rock; and Mr. G. W. Hawes, in his New Hampshire Report says (p. 187), of New Hampshire's "orthoclaseporphyry," "were it not that the feldspar is opaque orthoclase instead of clear sanidin [that is, glassy orthoclase] one would immediately think of trachyte on examining these rocks." Moreover, Messrs. E. Reyer and Suess, eminent geologists of Vienna, have shown that trachyte occurs in the Euganean Hills of Cretaceous and Jurassic age, as well as of Tertiary. Further, there are felsytes among the "younger" rocks of the globe, that is, among the products of volcanoes, where there is no trachyte; and on the other hand, trachyte sometimes graduates indefinitely into felsyte.

The facts show, consequently, that orthoclase rocks, or orthoclase and oligoclase, have been erupted from Paleozoic time onward, and that the distinctions found in some of the latest kinds are superficial: a little rougher surface, more translucency in the feldspar, and some glass at times among the crystalline grains; but nothing that has any geological weight. While then it may be well to retain the names of trachyte and felsyte, on account of the obvious external differences and the

wide extent to which the two varieties of rock are distributed over the earth's surface, the epithet "younger" as applied to trachyte and some felsyte can subserve plainly no good use.

The essential chemical identity of the "older" and "younger" rocks is further exhibited in the fact that the hornblendebearing rock labradorite-dioryte, called one of the "older," has the same ultimate constitution as the augite-bearing rocks "older" and "younger," called diabase, doleryte and basalt. This fact emphasizes the great truth, that the rock-making materials of former time are the same as those of recent.

During and since the Tertiary era more true subaerial volcanic eruptions have taken place than in any one ancient period; but there were also many then. As to fundamental differences between the materials ejected by the "older" and younger" world there appear to be none which are of essential importance. Glass or no glass is made an important criterion; but glass is simply a result of comparatively rapid cooling and alone indicates no essential differences in the melted mass.

Dropping the adjectives "younger" and "older" would require the dropping of the distinctive names based on them, unless some better reason exists for retaining them.

If diabase is not distinct from doleryte in some important way besides that of time of eruption, the name diabase (the newer of the two) is unnecessary. In fact, the rocks are not distinct in external characters any more than in chemical or mineralogical. The rock of the Giant's Causeway was pronounced diabase on microscopic grounds when its geological age was unknown; but it has since been proved to be Miocene Tertiary; and now although just as much diabase in constitution as before, it becomes, on the "younger" and "older" scale, dolery te or basalt.

Some of the differences attributed to difference in age may be due to differences in origin-that is, to the rock's being metamorphic in one case, and eruptive in another. There are distinctions of this kind of great interest yet to be followed out; and they may sometimes have a sufficient geological value for recognition in distinct names, although this may not be generally the case.

2. Foliated or not.-Some rocks are described as having foliated pyroxene or foliated hornblende, that is, diallage, pseudohypersthene or smaragdite, as the characterizing ingredient. The question here is whether the distinction of foliated or not foliated is of sufficient importance to be used as a distinction among kinds of rocks.

In the first place it is trivial as a crystallographic distinction. Secondly, although mineralogy once made much of the distinc