392 GEOLOGY. 



been the cause, or one of the causes, of the subsidence. Coincident 

 with the downward warping of the trough, the adjacent land area which 

 supplied the sediments, was probably warping upward steadily or 

 periodically. 



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 Niagara epoch; (2) a partial withdrawal of the 

 sea from the same area during the Salina epoch; and (3) a possible 

 extension of the sea at the close of the Salina epoch. There were 

 doubtless many minor oscillations of level not distinctly recorded in 

 the rocks, or, at any rate, not yet distinctly determined. 



Of the sediments deposited along the Atlantic coast on the eastern 

 border of Appalachia during the Silurian period little is known. They 

 have not been identified at any point within the present land area 

 south of New England. From this it is inferred that the Silurian shore 

 south of New England was farther east than that of to-day, and that 

 the Trans- Appalachian Silurian strata are still beneath the sea, deeply 

 buried by younger formations. There is no certain knowledge, however, 

 that the Silurian seas did not cover some part of the present land area 

 east of the mountains. If so, such formations as were made have been 

 removed by erosion, or transformed by metamorphism so that they 

 have not been recognized, or lie buried beneath later formations. Far- 

 ther north, as in New England and beyond, the sediments of this period 

 seem to have accumulated partly in bays which affected the coast, and 

 partly along the border of the open sea. 



Outcrops. — The principal areas where the Niagaran series appears 

 at the surface are shown on the map, Fig. 174. A map showing the 

 outcrop of the system as a whole would not be very different (see Plate 

 I). Fig. 174 shows also the relations of the Silurian to older systems, 

 and hazards a conjecture as to their probable occurrence where now 

 concealed. The principles underlying the distribution of outcrops 

 have already been discussed (p. 252 et seq.). 



Former extent and general stratigraphy. — The present margins of 

 the several Silurian formations are not their original margins, for their 

 exposed parts have suffered erosion, and the erosion of dipping beds, 

 as already explained, shifts their outcrops. In some localities there 

 are certain reliable data for estimating the former extension of the 



