356 



HISTORICAL GEOLOGY 



adjacent to lakes Erie and Ontario, sediments continued to 

 be deposited. While part of the beds were laid down under 

 water, this was evidently not the water of the open sea. The 

 rocks (Salina beds) consist of shales and sandstones of reddish 

 and gray colors interbedded with seams of gypsum and rock 

 salt. The salty beds are covered by a peculiar limestone, parts 

 of which are valuable for the manufacture of hydraulic cement, 

 and in this limestone are found, not the Niagara fossils, but 

 peculiar arthropods (Fig. 370) and fishes 

 of types which are almost unknown in 

 strictly marine formations. 



The Silurian salt beds of New York have 

 long furnished a large part of the salt used 

 in this country. Wells have been bored 

 through the overlying strata into the salt 

 beds, and the salty water is pumped to the 

 surface. There the water is evaporated and 

 the salt remains. 



At the present time beds of salt and 

 gypsum are produced in excessively salt 

 lakes, such as Great Salt Lake and the 

 FIG 370 A large ar- Dead Sea. These saline lakes are con- 

 thropod related to those fined to desert regions where evaporation 

 is ra P id - Tt is significant also that the 

 sediments deposited in some desert basins 

 are of a red or brownish color. From these considerations it 

 appears that, in the late Silurian, northeastern United States 

 had a distinctly arid climate. Most deserts are now situated 

 in the interiors of continents, either where they are sheltered 

 from moist winds by barrier mountain ranges, or where drying 

 winds, like the trade winds, blow constantly. The emergence 

 of the continent which seems to have occurred in the late 

 Silurian largely increased the area of land, and, if highlands of 

 sufficient elevation were so situated as to exclude the moist 

 winds from the Gulf of Mexico and the Atlantic, which now 

 bring rain to the Ontario region, the conditions for local 



