The sediments which gathered in the mediterranean seas were very thick, reaching in places the enormous depth of ten thousand feet.  Hence the bottoms of these seas were sinking troughs, ever filling with waste from the adjacent land as fast as they subsided.

Late Cambrian geography.  The formations of the Middle and Upper Cambrian are found resting unconformably on the pre-Cambrian rocks from New York westward into Minnesota and at various points in the interior, as in Missouri and in Texas.  Hence after earlier Cambrian time the central land subsided, with much the same effect as if the Mississippi valley were now to lower gradually, and the Gulf of Mexico to spread northward until it entered Lake Superior.  The Cambrian seas transgressed the central land and strewed far and wide behind their advancing beaches the sediments of the later Cambrian upon an eroded surface of pre-Cambrian rocks.

The succession of the Cambrian formations in North America records many minor oscillations and varying conditions of physical geography; yet on the whole it tells of widening seas and lowering lands.  Basal conglomerates and coarse sandstones which must have been laid near shore are succeeded by shaly sandstones, sandy shales, and shales.  Toward the top of the series heavy beds of limestone, extending from the Blue Ridge to Missouri, speak of clear water, and either of more distant shores or of neighboring lands which were worn or sunk so low that for the most part their waste was carried to the sea in solution.

In brief, the Cambrian was a period of submergence.  It began with the larger part of North America emerged as great land masses.  It closed with most of the interior of the continental plateau covered with a shallow sea.

THE LIFE OF THE CAMBRIAN PERIOD

It is now for the first time that we find preserved in the offshore deposits of the Cambrian seas enough remains of animal life to be properly called a fauna.  Doubtless these remains are only the most fragmentary representation of the life of the time, for the Cambrian rocks are very old and have been widely metamorphosed.  Yet the five hundred and more species already discovered embrace all the leading types of invertebrate life, and are so varied that we must believe that their lines of descent stretch far back into the pre-Cambrian past.

Plants.  No remains of plants have been found in Cambrian strata, except some doubtful markings, as of seaweed.

Sponges.  The sponges, the lowest of the multicellular animals, were represented by several orders.  Their fossils are recognized by the siliceous spicules, which, as in modern sponges, either were scattered through a mass of horny fibers or were connected in a flinty framework.

COELENTERATES.  This subkingdom includes two classes of interest to the geologist,—­the Hydrozoa, such as the fresh-water hydra and the jellyfish, and the corals.  Both classes existed in the Cambrian.