THE SILURIAN PERIOD. 



385 



merits to the sea lying west of it. This was probably because the land 

 was low, and it may have been low either because it had sunk or because 

 it had been nearly base-leveled in the preceding epochs. The slight 

 thickness of limestone in the east (New York) as compared with the 

 west (Wisconsin) remains to be accounted for. If the Niagara of the 

 west, as commonly classified, includes beds which are the time-equiva- 

 lents of the Clinton of the east, this would help to account for the greater 

 thickness of the Niagara of the west. Again, it may be that the sorts 

 of life which produce limestone were more abundant in the west, a 





Fig. 179a. — Photograph of a section of a chert nodule from the Niagara limestone 

 near Chicago, showing sponge structure. (From G. F Harris.) 



suggestion which the coral reefs, as well as other fossils of that region, 

 strengthen. 



The Guelph dolomite (Fig. 180) is distinguished from the Lockport 

 dolomite in western New York by the presence of a different fauna. 

 This fauna is, however, not restricted to beds overlying the Lockport 

 dolomite, but occurs as represented in Fig. 180. This section appears 

 to mean that the locality where it occurs was near the border of two 

 regions inhabited by different but contemporaneous faunas, and that 

 these faunas alternately had the advantage in the struggle for dom- 

 inance. 1 If this interpretation be correct, the Guelph and Lockport 

 dolomites are, in part at least, contemporaneous. 



While the Niagaran beds of the interior are in general nearly hor- 

 izontal, they are frequently domed so as to give the beds a high angle 



1 New York State Museum, Memoir 5, 1903, pp. 4-22. 



