been discovered in Canada are thought to come from rocks which are probably of Huronian age. In Bavaria, Dr Gümbel has described a species of Eozoön under the name of Eozoön Bavaricum, from certain metamorphic limestones which he refers to the Huronian formation. Lastly, the late Mr Billings described, from rocks in Newfoundland apparently referable to the Huronian, certain problematical limpet-shaped fossils, to which he gave the name of Aspidella.

LITERATURE.

Amongst the works and memoirs which the student may consult with regard to the Laurentian and Huronian deposits may be mentioned the following:*

(1) Report of Progress of the Geological Survey of Canada from its Commencement to 1863,' pp. 38-49, and pp. 50-66.

(2) Manual of Geology.' Dana. 2d Ed. 1875.

(3) 'The Dawn of Life.' J. W. Dawson. 1876.

(4)

"On the Occurrence of Organic Remains in the Laurentian Rocks of Canada." Sir W. E. Logan. Quart. Journ. Geol. Soc.,' xxi. 45-50.

(5) "On the Structure of Certain Organic Remains in the Laurentian Limestones of Canada." J. W. Dawson. 'Quart. Journ. Geol. Soc.,' xxi. 51-59.

(6)

"Additional Note on the Structure and Affinities of Eozoön Canadense." W. B. Carpenter. 'Quart. Journ. Geol. Soc.,' xxi. 59-66.

(7) "Supplemental Notes on the Structure and Affinities of Eozoön Canadense." W. B. Carpenter. 'Quart. Journ. Geol. Soc.,

xxii. 219-228.

(8) "On the So-Called Eozoönal Rocks."

Journ. Geol. Soc.,' xxii. 185-218.

King & Rowney. 'Quart.

(9) Chemical and Geological Essays.' Sterry Hunt.

The above list only includes some of the more important memoirs which may be consulted as to the geological and chemical features of the Laurentian and Huronian Rocks, and as to the true nature of Eozoön. Those who are desirous of studying the later phases of the controversy with regard to Eozoon must consult the papers of Carpenter, Carter, Dawson, King & Rowney, Hahn, and others, in the Quart. Journ. of the Geological Society,' the 'Proceedings of the Royal Irish Academy,' the 'Annals of Natural History,' the 'Geological Magazine,' &c. Dr Carpenter's Introduction to the Study of the Foraminifera ' should also be consulted.

* In this and in all subsequently following bibliographical lists, not only is the selection of works and memoirs quoted necessarily extremely limited; but only such have, as a general rule, been chosen for mention as are easily accessible to students who are in the position of being able to refer to a good library. Exceptions, however, are occasionally made to this rule, in favour of memoirs or works of special historical interest. It is also unnecessary to add that it has not been thought requisite to insert in these lists the well-known handbooks of geological and paleontological science, except in such instances as where they contain special information on special points.

CHAPTER VIII.

THE CAMBRIAN PERIOD.

The traces of life in the Laurentian period, as we have seen, are but scanty; but the Cambrian Rocks—so called from their occurrence in North Wales and its borders ("Cambria")—have yielded numerous remains of animals and some dubious plants. The Cambrian deposits have thus a special interest as being the oldest rocks in which occur any number of well-preserved and unquestionable organisms. We have here the remains of the first fauna, or assemblage of animals, of which we have at present knowledge. As regards their geographical distribution, the Cambrian Rocks have been recognised in many parts of the world, but there is some question as to the precise limits of the formation, and we may consider that their most typical area is in South Wales, where they have been carefully worked out, chiefly by Dr Henry Hicks. In this region, in the neighbourhood of the promontory of St David's, the Cambrian Rocks are largely developed, resting upon an ancient ridge of PreCambrian (Laurentian ?) strata, and overlaid by the lowest beds of the Lower Silurian. The subjoined sketch-section (fig. 27) exhibits in a general manner the succession of strata in this locality.

From this section it will be seen that the Cambrian Rocks in Wales are divided in the first place into a lower and an upper group. The Lower Cambrian is constituted at the base by a great series of grits, sandstones, conglomerates, and slates, which are known as the "Longmynd group," from their vast development in the Longmynd Hills in Shropshire, and which attain in North Wales a thickness of 8000 feet or more. The Longmynd beds are succeeded by the so-called "Menevian group," a series of sandstones, flags, and grits, about 600 feet in thickness, and containing a considerable number of fossils. The Upper Cambrian series consists in its lower portion of nearly 5000 feet of strata, principally shaly and slaty, which are known as the "Lingula Flags," from the great abundance in them of a shell referable to the genus Lingula. These are followed by 1000 feet of dark shales and flaggy sandstones, which are known as the "Tremadoc slates," from their occurrence near Tremadoc in North Wales; and these in turn are surmounted, apparently quite conformably, by the basement beds of the Lower Silurian.

The above may be regarded as giving a typical series of the Cambrian Rocks in a typical locality; but strata of Cambrian age are known in many other regions, of which it is only possible here to allude to a few of the most important. In Scandinavia occurs a well-developed series of Cambrian deposits, representing both the lower and upper parts of the

formation. In Bohemia, the Upper Cambrian, in particular, is largely developed, and constitutes the so-called "Primordial zone of Barrande. Lastly, in North America, whilst the Lower Cambrian is only imperfectly developed, or is represented by the Huronian, the Upper Cambrian formation has a wide extension, containing fossils similar in character to the analogous strata in Europe, and known as the "Potsdam Sandstone." The subjoined table shows the chief areas where Cambrian Rocks are developed, and their general equivalency: TABULAR VIEW OF THE CAMBRIAN FORMATION.

Like all the older Palæozoic deposits, the Cambrian Rocks, though by no means necessarily what would be called actually "metamorphic," have been highly cleaved, and otherwise altered from their original condition. Owing partly to their indurated state, and partly to their great antiquity, they are usually found in the heart of mountainous districts, which have undergone great disturbance, and have been subjected to an enormous amount of denudation. In some cases, as in the Longmynd Hills in Shropshire, they form low rounded elevations, largely covered by pasture, and with few or no elements of sublimity. In other cases, however, they rise into bold and rugged mountains, girded by precipitous cliffs. Industrially, the Cambrian Rocks are of interest, if only for the reason that the celebrated Welsh slates of Llanberis are derived from highly-cleaved beds of this age. Taken as a whole, the Cambrian formation is essentially composed of arenaceous and

muddy sediments, the latter being sometimes red, but more commonly nearly black in colour. It has often been supposed that the Cambrians are a deep-sea deposit, and that we may thus account for the few fossils contained in them; but the paucity of fossils is to a large extent imaginary, and some of the Lower Cambrian beds of the Longmynd Hills would appear to have been laid down in shallow water, as they exhibit rain-prints, sun-cracks, and ripple-marks-incontrovertible evidence of their having been a shore-deposit. The occurrence of innumerable worm-tracks and burrows in many Cambrian strata is also a proof of shallow-water conditions; and the general absence of limestones, coupled with the coarse mechanical nature of many of the sediments of the Lower Cambrian, may be taken as pointing in the same direction.

The life of the Cambrian, though not so rich as in the succeeding Silurian period, nevertheless consists of representatives of most of the great classes of invertebrate animals. The coarse sandy deposits of the formation, which abound more particularly towards its lower part, naturally are to a large extent barren of fossils; but the muddy sediments, when not too highly cleaved, and especially towards the summit of the group, are replete with organic remains. This is also the case, in many localities at any rate, with the finer beds of the Potsdam Sandstone in America. Limestones are known to occur in only a few areas (chiefly in America), and this may account for the apparent total absence of corals. It is, however, interesting to note that, with this exception, almost all the other leading groups of Invertebrates are known to have come into existence during the Cambrian period.

Of the land - surfaces of the Cambrian period we know nothing; and there is, therefore, nothing surprising in the fact that our acquaintance with the Cambrian vegetation is confined to some marine plants or sea-weeds, often of a very obscure and problematical nature. The "Fucoidal Sandstone" of Sweden, and the "Potsdam Sandstone" of North America, have both yielded numerous remains which have been regarded as markings left by sea-weeds or "Fucoids;" but these are highly enigmatical in their characters, and would, in many instances, seem to be rather referable to the tracks and burrows of marine worms. The first-mentioned of these formations has also yielded the curious, furrowed and striated stems which have been described as a kind of land-plant under the name of Eophyton (fig. 28). It cannot be said, however, that the vegetable origin of these singular bodies has been satisfactorily proved. Lastly, there are found in certain green and purple