Page:The Habitat of the Eurypterida.djvu/101

 seems to be unnecessary. The annelids surely would be more naturally accounted for as terrestrial forms and besides, it would scarcely be possible for them to leave their trails in deposits formed under water. Such trails would have to be made on surfaces exposed to the air long enough to harden and to be covered by wind-blown sand or dust or by a fresh deposit of water-laid material, but in the latter case a sufficient length of time would have to elapse to allow of the thorough hardening of the trail. In this case as in many another the question must be raised: Why if the eurypterids were marine were they the only organisms which were carried in from the open sea? It is well known that the littoral waters of the Pre-Cambric must have teamed with all the forms of life which are so abundantly represented in the advancing Cambric waters. It seems absurd to suppose that thousands of fragments of a eurypterid should have been washed in from the sea, but no other marine form.

The great thicknesses of Algonkian limestone found in the Belt terrane and corresponding formations have been adequately accounted for by Walcott as algal deposits in a series of lakes formed within the Cordilleran geosyncline. "The lakes of Algonkian [Pre-Cambric ] time were not much if any larger in area than the 'Great Lakes' of the St. Lawrence drainage basin and they were much shallower and more laden with mud and mineral matter in solution.

"The area of the Belt terrane in Montana is about 6000 square miles. This seems large when studying it in the field, but it is only one-fifth of the size of our great fresh-water Lake Superior" (290, 89).

Walcott has described nine species of calcareous algae from the Newland limestone below the Greyson shales and one which is abundant in the Spokane shales just above the Greyson. It is much more logical to suppose that the Greyson shales represent river rather than marine deposits, for they are coarse and arenaceous with interbedded shales, in which algal reefs could not grow. This would account for the absence of the reefs in the Greyson and for the absence of the eurypterids in the Newland limestone. I make this suggestion merely as a more plausible explanation of the conditions than the one which is usually offered.

If it can be assumed as proved that the remains in the Belt Terrane are of eurypterid affinity, they would offer just the proof