Page:Walcott Cambrian Geology and Paleontology II.djvu/33

 by a sinking of the ocean bed that lowered the shore line of all the continents. It was of world-wide extent and of great duration, and it wasduring this period that the open-sea fauna, as described by Brooks, probably found its way to the littoral zone and developed in the protected waters along the ancient continental shelves. Of this period we have no known record either in marine sedimentation or in life.

The evidence afforded by the few traces of pre-Cambrian fossils is inconclusive as far as determining whether their habitat was in marine, brackish, or fresh water.

The fossils from the Chuar group of Arizona [Walcott, 1899, pl. 23, figs. 1-4; pl. 27, figs. 9-13] are not sufficiently characterized to prove their origin or habitat. The ''Protozoan, Cryptozoon ? occidentale'' Dawson, is very abundant in Arizona, also in the Belt series of Montana, Alberta, and British Columbia. It occurs in limestones similar to those deposited in the fresh-water lagoons of Florida, and similar to the limestones of the lake deposits of the Tertiary formations of the Great Plains region of North America. The fossils of the Beltina zone of Montana and Alberta could as readily have been developed in fresh or brackish waters. There is nothing about the crustacean remains incompatible with their living in fresh water, in fact, the fragments indicate a form more nearly related to the fresh-water Branchiopoda with very thin test, rather than the strong Merostome (Eurypterus, etc.).

The oldest Cambrian fauna now known, with Nevadia wecksi and Holmia rowei [Walcott, 1910, pp. 257 and 292], is limited to a few forms, but with a careful examination of the region where it occurs in southwestern Nevada it is highly probable that a considerable fauna will be found. The strata in which it occurs were deposited in a depression opening out toward the Pacific ocean, where southwestern California is now located; this depression soon extended northward and presumably connected through to British Columbia and Alberta, as the same species of Olenellus occur in the central and upper portions of the Lower Cambrian both in Nevada and Alberta.

I do not know of a Cambrian fauna as old as that of Nevadia wecksi on the eastern side of the continent, or on the European continent. It appears to be a portion of the older fauna that is missing everywhere except in southwestern Nevada. I think it was brought in by the advancing Lower Cambrian sea from a sea to