544 GEOLOGY 



the age of the beds is open to question. Soje of them may be 

 Silurian. If the Silurian is really absent from all the areas where 

 its presence is not now known, it would appear that a large part of 

 western North America was land during the Silurian period. Silu- 

 rian beds are however known in Southern California, Nevada, Utah, 

 and Alaska, and perhaps in the Canadian Rockies, and its distri- 

 bution may be more widespread than has been supposed. 



Summary 



As in the case of all preceding systems of the Paleozoic, the 

 greatest thicknesses of Silurian strata (estimated at about 5,000 

 feet, maximum) occur in the Appalachian mountain region. Over 

 the interior, the system is relatively thin, being measured by hun- 

 dreds of feet rather than thousands. In keeping with these varia- 

 tions, the system is largely of clastic sediments of shallow-water 

 origin in the Appalachian belt, while in the interior it is largely of 

 limestone. The site of sedimentation in the east was a sort of 

 trough (the Appalachian trough) shut off from free communication 

 with the interior sea, but connected with the Atlantic, perhaps by 

 way of the present Chesapeake region. Since most of the sediments 

 of this trough were deposited in shallow-water, they are usually 

 thought to indicate that the trough was sinking at a rate comparable 

 to that at which the sediments accumulated. This view may, 

 however, need to be qualified as suggested on page 461. So far as 

 sinking took place, the thick sediments may have been its cause, 

 or one of its causes. With the down-warping of the trough, there 

 may have been up-warping of the adjacent land area which supplied 

 the sediments. 



The history of the Silurian period, as now understood, involves, 

 (1) a general submergence of the eastern part of the United States 

 west of Appalachia, by which the sea became more and more wide- 

 spread until the close of the Niagaran epoch; (2) a partial with- 

 drawal of the sea from the same area during the Salina epoch; and 

 (3) an extension of the sea at the close of that epoch. There were 

 doubtless many minor oscillations of level which have not been 

 determined. 



Former extent and general stratigraphy. The present margins 

 of the several Silurian formations are not their original margins, 



