The Elements of Geology

by William Harmon Norton

The first three systems were named from the fact that their strata are well displayed in Wales. The Cambrian carries the Roman name of Wales, and the Ordovician and Silurian the names of tribes of ancient Britons which inhabited the same country. The Devonian is named from the English county Devon, where its rocks were early studied. The Carboniferous was so called from the large amount of coal which it was found to contain in Great Britain and continental Europe.

The Cambrian

Distribution of strata. The Cambrian rocks outcrop in narrow belts about the pre-Cambrian areas of eastern Canada and the Lake Superior region, the Adirondacks and the Green Mountains. Strips of Cambrian formations occupy troughs in the pre-Cambrian rocks of New England and the maritime provinces of Canada; a long belt borders on the west the crystalline rocks of the Blue Ridge; and on the opposite side of the continent the Cambrian reappears in the mountains of the Great Basin and the Canadian Rockies. In the Mississippi valley it is exposed in small districts where uplift has permitted the stripping off of younger rocks. Although the areas of outcrop are small, we may infer that Cambrian rocks were widely deposited over the continent of North America.

Physical geography. The Cambrian system of North America comprises three distinct series, the Lower Cambrian, the Middle Cambrian, and the Upper Cambrian, each of which is characterized by its own peculiar fauna. In sketching the outlines of the continent as it was at the beginning of the Paleozoic, it must be remembered that wherever the Lower Cambrian formations now are found was certainly then sea bottom, and wherever the Lower Cambrian are wanting, and the next formations rest directly on pre-Cambrian rocks, was probably then land.

Fig. 264. Hypothetical Map of Eastern North America at the Beginning of Cambrian Time
Unshaded areas, probable land

Early Cambrian geography. In this way we know that at the opening of the Cambrian two long, narrow mediterranean seas stretched from north to south across the continent. The eastern sea extended from the Gulf of St. Lawrence down the Champlain-Hudson valley and thence along the western base of the Blue Ridge south at least to Alabama. The western sea stretched from the Canadian Rockies over the Great Basin and at least as far south as the Grand Canyon of the Colorado in Arizona.

Between these mediterraneans lay a great central land which included the pre-Cambrian U-shaped area of the Laurentian peneplain, and probably extended southward to the latitude of New Orleans. To the east lay a land which we may designate as Appalachia, whose western shore line was drawn along the site of the present Blue Ridge, but whose other limits are quite unknown. The land of Appalachia must have been large, for it furnished a great amount of waste during the entire Paleozoic era, and its eastern coast may possibly have lain even beyond the edge of the present continental shelf. On the western side of the continent a narrow land occupied the site of the Sierra Nevada Mountains.

Thus, even at the beginning of the Paleozoic, the continental plateau of North America had already been left by crustal movements in relief above the abysses of the great oceans on either side. The mediterraneans which lay upon it were shallow, as their sediments prove. They were epicontinental seas; that is, they rested upon (Greek, epi) the submerged portion of the continental plateau. We have no proof that the deep ocean ever occupied any part of where North America now is.