Page:EB1911 - Volume 05.djvu/100

Rh former a barren series of conglomerates and quartzites, the latter a series of grey and green fissile shales 1200 ft. thick with sandstones, greywackes and conglomerates.

Scandinavia.—Here the Cambrian system is only distinguished clearly on the eastern side, where the three subdivisions are found in a thin series of strata (400 ft.), in which black concretion-bearing shales play an important part. Limestones and shales with the Euloma-Niobe fauna come at the top. The upper series (Olenus) has been minutely zoned by W. C. Brögger, S. A. Tullberg and J. C. Moberg. In the middle series (Paradoxides) three thin limestone bands have been distinguished, the Fragmenten-Kalk, the Exulans-Kalk and the Andrarums-Kalk.

On the Norwegian side the Cambrian is perhaps represented by the Röros schists which lie at the base of a great series of crystalline schists, the probable equivalent of Ordovician and Silurian rocks.

Baltic Province.—The Cambrian rocks in this region are nearly all soft sediments, some 600 ft. thick; they reach from the Gulf of Finland towards Lake Ladoga. At the base is the so-called “blue clay” (really greenish) with ferruginous sandstones and with a fucoidal sandstone at its summit. This division is the equivalent of the Lower Cambrian. Above the fucoidal sandstone an important break appears in the system, for the Paradoxides and Olenus divisions are absent. The upper members are the “Ungulite grit” and about 20 ft. of Dictyonema shale. Cambrian rocks have been traced into Siberia (lat. 71°) and on the island of Vaigatch.

Central Europe.—Besides the Bohemian region previously mentioned, Cambrian rocks are present in Belgium and the north of France, in Spain and the Thüringer Wald. In the Ardennes the system is represented by grits and sandstones, shales, slates and quartz schists, and includes also whet slates and some igneous rocks. A. Dumont has arranged the whole series (Terrain ardennais) into three systems, an upper “Salmien,” a middle “Revinien” and a lower “Devillien,” but J. Gosselet has subsequently proposed to unite the two lower groups in one.

France.—In northern France Cambrian rocks, mostly purple conglomerates and red shales, rest with apparent unconformability upon pre-Cambrian strata in Brittany, Normandy and northern Poitou. In the Rennes basin limestones—often dolomitic—are associated with quartzites and conglomerates; silicious limestones also occur in the Sarthe region. Farther south, around the old lands of Languedoc, equivalents of the two upper divisions of the Cambrian have been recorded; and the uppermost members of the system appear in Herault. Patches of Cambrian rocks are found in the Pyrenees.

In Spain slates and quartzites, the slates of Rivadeo, more than 9000 ft. thick, are followed by the middle Cambrian beds of La Vega, thick quartzites with limestone, slates and iron ores. Cambrian rocks occur also in the provinces of Seville and Ciudad-Real. Upper Cambrian strata have been found in upper Alemtejo in Portugal.

In Russian Poland is a series of conglomerates, quartzites and shales; some of the beds yield a Paradoxides fauna.

In the Thüringer Wald are certain strata, presumably Cambrian since the uppermost beds contain the Euloma-Niobe fauna.

Sardinia contains both middle and upper Cambrian. The Cambrian system is represented in the Salt Range of India by the Neobolus or Khussack beds, which may possibly belong to the middle subdivision. The same group is probably represented in Corea and the Liao-tung by the thick “Sinisian” formation of F. von Richthofen.

In South America upper Cambrian rocks have been recorded from north Argentina.

The Lower Cambrian has been found at various places in South Australia; and in Tasmania a thick series of strata appears to be in part at least of Upper Cambrian age.

General Physical Conditions in the Cambrian Period.—The Cambrian rocks previously described are all such as would result from deposition, in comparatively shallow seas, of the products of degradation of land surfaces by the ordinary agents of denudation. Evidences of shallow water conditions are abundant; very frequently on the bedding surfaces of sandstones and other rocks we find cracks made by the sun’s heat and pittings caused by the showers that fell from the Cambrian sky, and these records of the weather of this remote period are preserved as sharply and clearly as those made only to-day on our tidal reaches. Ripple marks and current bedding further point to the shallowness of the water at the places where the rocks were made.

No Cambrian rocks are such as would be formed in the abysses of the sea—although the absence of well-developed eyes in the trilobites has led some to assume that this condition was an indication that the creatures lived in abyssal depths.

At the close of the pre-Cambrian, many of the deposits of that period must have been elevated into regions of fairly high ground; this we may assume from the nature of the Cambrian deposits which are mainly the product of the denudation of such ground. Over the land areas thus formed, the seas in Cambrian time gradually spread, laying down first the series known as Lower Cambrian, then by further encroachment on the land the wider spread Upper Cambrian deposits—in Europe, the middle series is the most extensive. Consequently, Cambrian strata are usually unconformable on older rocks.

During the general advance of the sea, local warpings of the crust may have given rise to shallow lagoon or inland-lake conditions. The common occurrence of red strata has been cited in support of this view.

Compared with some other periods, the Cambrian was free from extensive volcanic disturbances, but in Wales and in Brittany the earlier portions of this period were marked by voluminous outpourings; a condition that was feebly reflected in central and southern Europe.

No definite conclusions can be drawn from the fossils as to the climatic peculiarities of the earth in Cambrian times. The red rocks may in some cases suggest desert conditions; and there is good reason to suppose that in what are now Norway and China a glacial cold prevailed early in the period.

Considerable variations occur in the thickness of Cambrian deposits, which may generally be explained by the greater