UPPER SILURIAN. 



219 



central and western New York, Ohio, and Wisconsin ; also along the 

 Appalachians, from Pennsylvania to Alabama ; also in Nova Scotia. 



The rocks of the Niagara epoch are among the most extensive of 

 the continent, occurring over a large part of the Continental Interior, 

 from New York westward and southwestward; in the Eastern Border 

 region, on Anticosti ; and in the Arctic and other parts of British 

 America. In all these regions, they are partly or wholly limestone, 

 the Niagara having been, like the Trenton, one of the limestone-making 

 epochs of North America. Near Niagara Falls, there are 1 65 feet of 

 limestone resting on 80 of shale; and directly at the fall, 85 of lime- 

 stone over the 80 of shale ; and the removal of the shale by the 

 waters is the occasion of the slow retrocession of the falls. Along the 

 Appalachians, the rocks have a thickness of 1,500 feet, and extend to 

 Alabama. 



In Illinois and Missouri, there are no shales or sandstones interven- 

 ing between the limestones of the Cincinnati and Niagara eras ; and, 

 as the two formations are continuous, it may be that the Medina and 

 Clinton epochs are there represented by limestone. 



1. Medina Epoch (5 a). 



The relation of the Medina group to the 

 Fig. 396. overlying Clinton and Niagara groups is well 



illustrated in one or two sections from the 

 western part of the State of New York, 

 Fig. 396 represents the rocks at Genesee 

 Falls, near Rochester. The lower strata, 1, 

 2, are the Medina sandstone (5 b); 3, 4, 5, 

 6, the Clinton group (5 c) ; and 7, 8, the 

 Niagara group (5 d), — 2 being a grit rock, 

 3 and 5 shales, 4 and 6 limestone, 7 shale, 

 and 8 limestone. The whole height is about 400 feet. 



The following figure (397) represents a section of the rocks along Niagara River, 

 from the bluff at Lewiston (L) to the Falls at F, passing by the Whirlpool at W, — a 

 distance of seven miles. 



In the beds at Lewiston, there are eight strata: 1, 2, 3, 4 belong to the Medina group, 

 and consist — 1 and 3, of shaly sandstone; 2 and 4, of hard sandstone; 5, of shale, and 



Section at Genesee Falls. 



Fiff. 397. 



Section along the Niagara, from the Falls to Lewiston Heights. 



6, limestone, are of the Clinton group; 7, a shale, and 8, limestone, of the Ni 

 group. The dip is up-stream, as in the figure, but is only fifteen feet to a mile. 



