250 PALEOZOIC TIME. 



It is barely possible that some of the Mollusks may have lived in 

 fresh waters ; but the remains are so mingled with species that are 

 obviously salt-water types that it cannot be proved to be true of any. 



2. Individuality of the Eastern-border region in American geo- 

 logical history. — Some general facts bearing on this subject are men- 

 tioned on page 160. The individuality of the region is illustrated 

 most conclusively by the life of the waters, as shown by Salter and 

 Billings. 



Thus, there are, in the beds of this region of the Primordial and Canadian periods, 

 Salterella ruyosa Billings, closely like the Scottish ; S. Maccullochi, Salter ; Kutorgina 

 cingulata B., said by Davidson and Hall to occur in the Lingula flags; Acrotreta 

 gemma B., very near A. subconica Kutorga; four species of Piloceras, a genus de- 

 scribed from Scotland, but not known in the United States; Holometojms AngeliniB. f 

 very near II. limbatus Angelin, of Sweden; Nileus macrops B., N. scrutatus B., N. 

 affinis B., all closely allied to N. armadillo of Dahnan ; Harpides Atlunticus. very near 

 Angelin's //. rugosus of Sweden. In the beds of Cincinnati group age, there are 

 Ascoceras Canadmse B., A. Newberryi B., and Glossoceras desideratum B., not found in 

 the United States. In the Upper Silurian, there are, as shown by Salter, the British spe- 

 cies Rhynchonella Wilsoni Sow., Grammysia triangulata Salter, G. cingidataWis., Platy- 

 schisma Helicites Sow., Acroculia Halioiis Sow., Bellerophon expansus Sow., B. carina- 

 tus Sow., 0. bullatum Sow. (?), 0. ibex Sow., Homalonotus Knightii Kiinig, Phacops 

 Downingii Salt., to which Billings adds Rhynchonella Stricklandii Sow., and Lituites 

 Americanum B., very near, if not quite identical with L. giganteum Sow. Mr. E. Bil- 

 lings, who furnished this work the above list of species, adds that, through the Pri- 

 mordial and Canadian periods, there is a decided European tinge in the life; but in 

 the Trenton period its character was peculiarly American. Then in the Cincinnati 

 epoch there was again a European tinge, which increased in strength through the Upper 

 Silurian. 



3. Conditions of the North American Continent. — The survey 

 of the successive formations of the Upper Silurian teaches that the 

 geological changes in progress, like those of the earlier Silurian, 

 operated widely over the continent. The causes in action were not 

 making a mere edging to the continent, as in Tertiary times, but were 

 building up the very continent itself by wide-spread accumulations of 

 limestone, sands, and clays. 



Moreover, the continental seas were not the ocean's bed, although 

 they may, over wide areas at times, have exceeded 500 or 1,000 

 fathoms in depth. In many of the rocks, the ripjjle-marks of some 

 layers, rill-marks of others, and cracks from sun-drying of others, 

 often in the same stratum, prove the shallowness of the water over 

 great regions, and a wide expanse of exposed beaches and marshes 

 elsewhere. The beds of iron ore in the Clinton group, which have 

 great extent, are other proof of wide-spread marshes over the coun- 

 try, since such deposits cannot form in the open sea. The brines of 

 the Salina period again mark a time of salt marshes or inland salt 

 lakes in New York. 



The continent still included comparatively little permanent dry 



