380 GEOLOGY. 



the land to emerge, or because the sea-bottom had been converted 

 into land by aggradation. 



A significant feature of the distribution of the Niagara formation 

 to be associated with its distribution in the interior of the continent 

 is its great development in high latitudes. It occurs in Manitoba, 

 and from here probably extends down into Minnesota. While this 

 area of Niagara is not known to be connected with that of Iowa and 

 Illinois, such connection is probable, being now obscured by the heavy 

 body of drift which covers most of Minnesota. The Niagara forma- 

 tion is also present west of Hudson Bay, and over numerous large 

 and small areas on the continent and on islands between the parallels 

 of 60° and 76°. It is also present in Grinnell Land and Greenland in 

 latitude 80°. 



The isolated areas of the Niagara formation in these northern 

 regions appear to be remnants of a once continuous body. This 

 inference is rendered probable by the similarity of the fossils through- 



Fig. 175. — Contact of the Niagara limestone and the Maquoketa (Hudson River 

 shale) in northeastern Iowa. The view illustrates the effect of inequalities of 

 hardness on topography. The cultivated surface with the gentle slope is under- 

 lain by Hudson River shale; the wooded slopes above are the escarpment of 

 Niagara limestone. (Calvin, Iowa Geol. Surv.) 



out the northern area. From the same line of evidence, it is inferred 

 that there was shallow water connection between the Mississippi basin 

 and northern Europe by way of the arctic islands, a connection which 

 permitted the intermigration of the shallow-water animals of the two 

 regions. 



Between the northern areas of exposed Niagaran, older formations 

 from which the Niagara has been removed are exposed. In the United 

 States, on the other hand, the more or less widely separated areas 



