Page:EB1911 - Volume 27.djvu/649

GEOLOGY] possible now, and may never be. The Proterozoic formations have yielded a few fossils in several places, especially Montana and northern Arizona; but they are so imperfect, their numbers, whether of individuals or of species, are so small, and the localities where they occur so few, that they are of little service in correlation throughout the United States. The carbon-bearing shales, slates and schists, and the limestone, are indications that life was relatively abundant, even though but few fossils are preserved. Among the known fossils are vermes, crustacean and probably brachiopods and pteropods.

The character of the sediments of the Proterozoic is such as to show that mature weathering affected the older rocks before their material was worked over into the Proterozoic formations. This mature weathering, resulting in the relatively complete separation of the quartz from the kaolin, and both from the calcium carbonate and other basic materials, implies conditions of rock decay comparable to those of the present time.

In all but a few places where their relations are known, the Proterozoic rocks are unconformable beneath the Palaeozoic. Where conformity exists the separation is made on the basis of fossils, it having been agreed that the oldest rocks carrying the Olenellus fauna are to be regarded as the base of the Cambrian system.

The Palaeozoic and later formations are usually less altered, more accessible, and better known than the Proterozoic and Archeozoic, and will be taken up by systems.



Cambrian System.—The lower part of the Cambrian system, characterized by the Olenellus fauna, is restricted to the borders of the continent, where it rests on the older rocks unconformable in most places. The middle part of the system, characterized by the Paradoxides fauna, is somewhat more widespread, resting on the lower part conformably, but overlapping it, especially in the south and west. The upper part of the system, carrying the Dicellocephalus fauna, is very much more extensive; it is indeed one of the most widespread series of rocks on the continent. The lower, middle and upper parts of the system all contain marine fossils. This being the case, the distribution of the several divisions indicates that progressive submergence of the United States was in progress during the period, and that most of the country was covered by the sea before its close.

The system is composed chiefly of elastic rocks, and their composition and structure show that the water in which they were deposited was shallow. In the interior, the upper part of the system, the Potsdam sandstone, is generally arenaceous. It is well exposed in New York, Wisconsin, Missouri and elsewhere, about the outcrops of older rocks. The system is also exposed in many of the western mountains or about their borders, especially about those the cores of which are of Archean or Proterozoic rock.

The thickness of the system has been estimated at 10,000 to

12,000 ft. in eastern New York, and almost as much in the southern Appalachian Mountains (Georgia and Alabama); but its average thickness is much less. In Wisconsin, where the Upper Cambrian only is present, the thickness is about 1000 ft. The greater thickness in the east appears to be due in part to the fact that an extensive area of land, Appalachia, lay east of the site of the Appalachian Mountains throughout the Palaeozoic era, and quantities of sediment from it were accumulated where these mountains were to arise later. The greatness of the thickness, as it has been measured, is also due in part to the oblique position in which the beds of sediment were originally deposited.

The Cambrian formations have not been notably metamorphosed, except in a few regions where dynamic metamorphism has been effective. The system is without any notable amount of igneous rock. As in other parts of the world, the system here contains abundant fossils, among which trilobites, brachiopods and worms are the most abundant. The range of forms, however, is great.

Ordovician System.—The succeeding Ordovician (Lower Silurian) system of rocks is closely connected with the Cambrian, geographically, stratigraphically and faunally. Its distribution is much the same as that of the Upper Cambrian, with which it is conformable in many places. The Ordovician system contains much more limestone, and therefore much less elastic rock, than the Cambrian, pointing to clearer seas in which life abounded. The succession of beds in New York has become a sort of standard with which the system in other parts of the United States has been compared. The succession of formations in that state is as follows:— The classification in the right-hand column of this table is not applicable in detail to regions remote from New York.

There is in some places an unconformity between the Richmond beds (or their equivalent) and underlying formations, and this unconformity, together with certain palaeontological considerations, has raised the question whether the uppermost part of the system, as outlined above, should not be classed as Silurian (Upper Silurian). Over the interior the strata are nearly horizontal, but in the mountain regions of the east and west, as well as in the mountains of Arkansas