The rocks older than the coal measures were for a long time heaped indiscriminately together, under the name of Greywacke, or Transition Rocks, and were little regarded by geologists.

About 1831, the problem of these ancients rocks was attacked by two eminent English geologists, Sedgwick and Murchison, who soon brought order out of the chaos. There was much discussion and dispute as to the limits of the systems into which the Greywacke should be divided, and as to the names which should be given to them. The oldest fossiliferous strata were by Sedgwick called Cambrian (from the Latin name for Wales), but were included by Murchison in his Lower Silurian. The latter example was long followed by most geologists, but the advance of knowledge has fully vindicated the claim of the Cambrian to rank as a distinct system. The divisions of the American Cambrian are as follows: -

3. Upper Cambrian, Saratogan Epoch, Dikellocephalus Fauna.

2. Middle Cambrian, Acadian Epoch, Paradoxides Fauna.

1. Lower Cambrian, Georgian Epoch, Olenellus Fauna.

American

In North America, Cambrian rocks are not exposed at the surface over large areas, being, for the most part, deeply buried under later sediments; their maximum thickness, so far as known, does not exceed 12,000 feet. While not forming extensive areas of the present surface, Cambrian strata are very widely distributed over the continent, usually resting uncom-formably upon the plicated and metamorphosed rocks of the Archaean and Algonkian. These strata are found in the pre-Cambrian depressions, from the Adirondacks to Newfoundland, and along the flank of the Appalachian uplift, from the St. Lawrence to Alabama. They also fringe Archaean or Algonkian areas in other regions, as in Wisconsin, Missouri, Texas, in the Rocky Mountain chain, from Colorado to British Columbia, and in the mountains of Nevada. Cambrian beds are exposed in the Colorado Canon, and doubtless would be found throughout the larger part of the continent were the overlying beds stripped away.

So far as they are accessible to observation, the Cambrian rocks are chiefly such as are laid down in shallow water near shore, conglomerates, sandstones, shales, which are ripple-marked in a way that betrays their shoal-water origin. There are also some areas of deeper water accumulations, found in the thick limestones of western Vermont, the Appalachian Mountains, Nevada, and British Columbia. Very little igneous rock is found in the Cambrian of North America. Small intrusions occur in Newfoundland and New England, and quite considerable ones in British Columbia, but some of these may be long post-Cambrian in date.

Fig. 259. - Map of known Cambrian outcrops in the United States and Canada.

As is indicated by the geographical distribution of the fossils, North America was, in the Cambrian period, divided into two provinces of very unequal size and faun ally very different. The Atlantic province, comprising Newfoundland, New Brunswick, Nova Scotia, and New England, shows so close a connection with Europe as to justify the inference that in high latitudes a land bridge spanned the Atlantic, or at least that a chain of islands and shoals permitted the migration of shore-loving marine animals from one continent to the other. All the rest of North America belongs to the Pacific province, though there are many local faunas within that vast area.

During Cambrian times the sea was slowly advancing over the land in North America, and the geography of the continent was very different at the close of the period from what it had been at the beginning. In the Lower Cambrian the land areas are inferred to have been somewhat as follows: First, there was the great northern mass of crystalline Archaean and Algonkian rocks, but this was probably much more extensive than the present exposures of pre-Cambrian rocks would indicate. It probably covered the whole Mississippi valley down to 300 N. lat. and extended westward beyond the Rocky Mountains. Long, narrow strips of land, alternating with narrow sounds, occupied part of New England and the maritime provinces of Canada, while an Appalachian land, whose western line is marked by the present Blue Ridge, extended eastward an unknown distance into the Atlantic. On the western shore of the Appalachian land was a narrow arm of the sea, which opened south and nearly separated this land area from the great mass of the continent. During the Lower and Middle Cambrian, this long and narrow bay or sound must have been closed, or only occasionally and partially opened, at the northern end. In later Cambrian times it was perhaps open.

The site of the Sierra Nevada was occupied by a long, narrow land, running from Puget Sound to Mexico, and another such area was found in eastern British Columbia. The Great Basin region was under water. Around these shores were laid down the coarser deposits of the Lower Cambrian, with great masses of shales and thick limestones in deeper water.

Middle Cambrian sediments have quite a similar distribution to those of the Lower, but the sea was slowly advancing over the continent from the south. Nothing is known of the condition of Central America and Mexico at this time, but from Arizona eastward to Alabama the land was submerged. This transgression of the sea continued, and reached its maximum in the Upper Cambrian. Toward the close of the period a large part of the continent had been submerged and, in particular, a vast interior sea had been established over the Mississippi Valley.