LOWER SILURIAN. 193 



stones. This is evident from the nature and distribution of the rocks. 

 The sandy limestones of the Calciferous appear to mark in New 

 York the transition from the Primordial (with its beach and sand-flat 

 formations) to the second or Canadian period, while the limestone, 

 along the Green Mountain range, if this is partly of the Chazy series, 

 shows that the water deepened in that direction. In Newfoundland, 

 on the other side of the same border region, the Calciferous epoch 

 was a time of immense limestone accumulations, their thickness 

 amounting to 3,200 feet ; while, in the later part of the period, there 

 were made as many feet of sandstones and shales, with little limestone. 

 Again, over the Mississippi basin, — a region of deeper waters 

 through a large part of geological histoiy, and much of the time the 

 southern half of an open "Mediterranean sea," connecting the At- 

 lantic (or Mexican Gulf) and the Arctic Ocean, — the rocks are very 

 largely limestones. Yet, even here, in some parts, sand-beds alter- 

 nate at intervals with the limestones, showing changes during the 

 period in the level of the sea-bottom or in the marine currents. 

 Toward the head of the basin, and about Lake Superior, the prev- 

 alence of sandstones proves that the waters were there shallow, as 

 Hall has remarked, and partly those of sea-shore flats and wind 

 drifts. The events prove that no one kind of rock was formed 

 simultaneously over a continent ; but that the several parts of the 

 continental seas were giving origin to different kinds, according to the 

 depth of water, nearness to sea-shores, the character of the currents, 

 and other circumstances. 



Billings has observed, with regard to the Quebec formation, that 

 the limestones of the era contain different fossils from the intervening 

 shales ; and yet both are essentially of the same age. The species 

 of clear waters are often wholly unlike the contemporaneous species 

 of muddy bottoms ; and hence a change of condition, from that 

 requisite for making limestones to that for shales, would naturally 

 be accompanied by a change of species, and then be followed by a 

 return of the former species, whenever (through some rising or sink- 

 ing of the sea-bottom or land) the seas returned to their former clear 

 condition. 



Origin of the limestones. — The limestones of New York and 

 Canada contain various fossils, and may have resulted from the trit- 

 uration of shells, crinoids, etc. If so, the species must have lived 

 in comparatively shallow water, like those making the shell banks or 

 coral reefs of the Pacific ; for the waves and currents are the pul- 

 verizing agents, and these, at great depths, are too feeble for such 

 work. 



The Lower Magnesian limestones of the Mississippi basin contain 

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