357 



deserts would have been present. The upper limestone, or 

 Water-lime formation, is thought to have been deposited 

 partly in fresh or brackish lakes, which were perhaps made 

 possible by an increase in the rainfall of that region. 



Closing incursion of the sea. In the vicinity of lakes 

 Erie and Ontario, where the Salina beds are best known, the 

 Water-lime formation grades upward into limestone with coral 

 reefs and marine shells. A second incursion of the epiconti- 

 nental sea is thus recorded. Between these Monroe strata, 

 as they are called, and the overlying Devonian rocks there is 

 no sharp dividing line, but merely a gradual change in the 

 kinds of fossils. 



QUESTIONS 



1. Why are the divisions of the Silurian system as recognized 

 in New York not suitable for Illinois ? 



2. The pebbles of the Oneida formation consist largely of pure 

 quartz. Can you suggest how pure quartz gravel could be derived 

 from a complex mass of igneous and metamorphic rocks such as 

 those which were exposed in the ancient continent of Appalachia ? 



3. By what process may loose gravel be transformed into a 

 hard rock capable of forming mountain ridges ? 



4. Why should fossils be rare in the Oneida formation, even if 

 shell-bearing animals were abundant at the time and place it was 

 deposited ? 



5. At Cobalt Lake, in northern Ontario, the Niagara lime- 

 stone lies directly upon the surface of Huronian and Archaean 

 igneous rocks. What different hypotheses may account for this 

 relation ? 



6. Small patches of Niagara limestone are found northeast of 

 the edge of the continuous 



formation in Canada and 



the United States. What 



is the significance of these 



outliers (Fig. 371), as they FlG - 371. Diagram of outliers. 



are called, with reference to the former distribution of the Silurian 



system ? 



