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. [Illustration: Fig. 265. Sponge Spicules as seen in Flint under the Microscope] =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. [Illustration: Fig. 266. Graptolites] The Hydrozoa were represented not only by jellyfish but also by the _graptolite_, which takes its name from a fancied resemblance of some of its forms to a quill pen. It was a composite animal with a horny framework, the individuals of the colony living in cells strung on one or both sides along a hollow stem, and communicating by means of a common flesh in this central tube. Some graptolites were straight, and some curved or spiral; some were single stemmed, and others consisted of several radial stems united. Graptolites occur but rarely in the Upper Cambrian. In the Ordovician and Silurian they are very plentiful, and at the close of the Silurian they pass out of existence, never to return.