THE SILURIAN PERIOD. 387 



ent. To the west, on the other hand, the Salina formation is re- 

 stricted to much narrower limits than the Niagara. Like the latter, 

 it underlies much of southern and southwestern New York, and prob- 

 ably western Pennsylvania as well, and extends west into Ohio. It 

 is also present in Ontario, and extends thence to the northern extremity 

 of the southern peninsula of Michigan, and to the eastern border of 

 Wisconsin, but it is not known in Illinois, or at any point farther west 

 in the basin of the Mississippi. If it now exists in these States, it 

 has not been identified. 



The absence of the Salina formation from so great an area where 

 the Niagara is present, points to the emergence of a considerable area 

 in the Mississippi basin at the close of the Niagara epoch, and the nature 

 of the Salina series, where present, shows that geographic relations 

 were greatly altered. A map showing the relations of land and water 

 during the Salina epoch would show a great increase in land over that 

 shown in Fig. 174. 



In the east, the Salina group embraces three principal varieties of 

 rock — limestone, shale, and rock salt. With these formations, some 

 gypsum, the usual concomitant of salt beds, is associated. Shales 

 constitute the larger part of the group, and seem to have originated 

 after the fashion of shales in general, but their meager content of 

 fossils seems to point to their deposition under conditions unfavorable for 

 life. 



The salt is widely distributed. It is known to occur at many 

 points in New York within an area 9,000 to 10,000 square miles in 

 extent, single beds of it being locally 40 to 80 feet in thickness. Several 

 beds of it sometimes occur one above another, interstratified with 

 other sorts of rock, and their aggregate thickness sometimes reaches 

 as much as 100 feet. In some sections, therefore, the salt makes a 

 very considerable fraction of the total thickness of the series. The 

 same relations hold still farther west. Near Cleveland, four salt beds, 

 50 feet and less in thickness, are interstratified with 500 feet of shales 

 belonging to this period. 



The presence of salt beds in New York and Ohio indicates that 

 during the movements which took place about the close of the Niagara 

 period, great lagoons or inclosed seas were formed. In these lagoons 

 it is believed the red shales and other formations of the Salina group 

 were deposited. Had the climate of the region been as moist as that 



