384 GEOLOGY. 



tions have shown that the conspicuous beds of chert nodules in the 

 dolomitic Niagara limestone of the Chicago area are in reality great 

 sponge beds. The minute spicular structure of these organisms may 

 be frequently detected under the microscope in thin sections of these 

 nodules (Fig. 179a), although in most of them these structures have 

 been nearly or wholly obliterated. 1 



East of the Appalachians and west of the Mississippi the distribu- 

 tion of strata of Niagara age is not generally known in detail. Lime- 



Fig. 179. — Oolitic limestone. 



stones which are perhaps referable to the Niagara occur in New 

 Hampshire and Maine, 2 and in the Provinces between these States and 

 the St. Lawrence. In Nova Scotia the formations of the Niagara 

 epoch are represented by 1300 feet of shales, affording another illus- 

 tration of the fact that very different sorts of rock may be accumulating 

 in different regions at the same time. 



Like the older formations of the Silurian, the Niagara has not been 

 identified with certainty in the western part of the continent. 



Unlike the preceding formations of the Silurian, the Niagara is 

 not thicker in the east than elsewhere. In the east, indeed, where the 

 formation is exposed it has a thickness of but 100 to 300 feet, while in 

 Wisconsin 3 it attains a maximum of 800 feet (probably including some 

 Clinton), all of which is limestone. It would appear that the land along 

 the eastern border of the continent was not yielding abundant sedi- 



1 G. F. Harris, personal communication. 



2 Williams, Bull. 165, U. S. Geol. Surv. 

 3 Geol. of Wis., Vols. I and II. 



