110 HISTORICAL GEOLOGY 



ward to Virginia in the Appalachians, New York, Ontario, Ohio, 

 and Michigan, and are quite generally characterized by red shales 

 and sandstones and salt and gypsum deposits. Such materials 

 imply arid climate conditions, with deposition in extensive lagoons 

 or more or less cut-off arms of the sea, rather than typical open 

 sea conditions. 



Since the immediately overlying Cayugan formations (Coble- 

 skill, Rondout, and Manlius) are mostly marine deposits and more 

 extensive than the Salina, it is evident that there was at least a 

 partial restoration of more widespread marine waters during later 

 Cayugan time. This later Cayugan sea spread from eastern New 

 York westward over the Salina lagoon areas and into eastern 

 Wisconsin, and from eastern New York southward through the 

 Appalachian district. So far as known the rest of the continent 

 was dry land. 



In addition to these broader and more important geographic 

 changes during the Silurian period, there were of course various 

 minor and generally local changes of relative level between land 

 and sea, some of these now being known and some not yet deter- 

 mined. 



Close of the Silurian. — At the close of the Silurian, or opening 

 of the Devonian, the Cayugan sea withdrew from the area from 

 central New York to Wisconsin, and but a few comparatively 

 small areas of eastern North America were submerged, as shown on 

 map (Fig. 69) . This was essentially the geography of the conti- 

 nent in earliest Devonian time and will be discussed in the next 

 chapter. 



There appear to have been no mountain-making (orogenic) 

 movements, and no important epeirogenic disturbances at the close 

 of the Silurian in North America. Because of the comparatively 

 quiet and gradual transition into the succeeding period, the Silurian 

 and Devonian systems are usually not sharply separated from each 

 other, and often, as in New York, there has been difficulty in 

 satisfactorily dividing the systems. 



Foreign Silurian 



The Ordovician division of Europe into two great provinces or 

 basins of deposition — northern and southern — was continued in 

 the Silurian, though the latter strata are not so widely distributed. 



