108 THE SILURIAN PERIOD 



the marine waters to a maximum in the mid-Silurian (Niagaran) 

 epoch. On the accompanying map (Fig. 60), the relations of land 

 and water are graphically depicted, and it will be seen that the 

 extent of marine waters was almost comparable to that of mid- 

 Ordovician time, though with the following chief differences in the 

 mid-Silurian: Appalachia was larger and higher because of the 

 Taconic uplift; the land areas east and west of Hudson Bay and in 

 the Cordilleran region were larger; and there was more land in 

 the Gulf border (especially Texas) region. That this Niagaran sea 

 was a true epicontinental or shallow water body is definitely known 

 for reasons similar to those given in the discussion of the mid- 

 Ordovician sea. 



In order to further impress upon the reader not only how a 

 marine transgression of this sort is proved, but also how the direc- 

 tion of encroachment can be determined, we may briefly consider 

 the excellent example afforded by the disposition of Silurian strata 

 in New York state. That central and western New York were 

 submerged before the Hudson Valley region is proved as follows. 

 In central New York the first deposit to form upon the eroded sur- 

 face of the Ordovician shales was the Oneida conglomerate. In 

 the Hudson Valley of New York (e.g. Shawangunk Range), the 

 first deposit to be laid down upon the eroded Ordovician shales 

 was the Shawangunk conglomerate which belongs with the Salina 

 division and is therefore much younger than the Oneida conglom- 

 erate. Also the Clinton and Niagara formations, well developed 

 in central and western New York, were never formed in the east- 

 ern or southeastern parts of the state, though they do extend 

 somewhat farther eastward than the Oneida. Thus we prove 

 that the Silurian sea overspread central and western New York 

 long before it reached the Hudson Valley region of southeastern 

 New York. 



Late Silurian. — Two prominent events mark the physical 

 history of late Silurian time, namely a very considerable with- 

 drawal of the extensive (Niagaran) sea in early Cayugan (Salina) 

 time, and a considerable, though only partial, reextension of the 

 sea in later Cayugan time. That a very appreciable retrogression 

 of the Niagaran sea ushered in Salina time is proved by both the 

 comparatively restricted distribution and the character of the 

 Salina strata. Thus in the eastern United States and Canada 

 Salina strata occur only through parts of Pennsylvania and south- 



