THE SILURIAN PERIOD 



441 



gravel and sand were carried by rapid streams flowing from the areas 

 newly raised by the Taconic deformation (p. 422) and were spread 

 along the shores, forming wide beaches, the gravel (forming the 

 Oneida conglomerate) rapidly thinning toward the west. Sand (Me- 

 dina) was carried farther out into the sea by the currents and formed 

 extensive sandstone 

 When in the 



strata, 

 course 

 lands 

 down, 



of time the 



were worn 



the streams 

 were unable to carry 

 such large quantities 

 of coarse sediment as 

 formerly, and the belt 

 of gravel along the 

 shores was conse- 

 quently narrowed 

 and sand was de- 

 posited nearer shore 

 and upon the earlier 

 gravels. It is evi- 

 dent from the above 

 that the conglomer- 

 ates and sandstones 

 were contempora- 

 neous. A still greater 

 lowering of the land, 

 either by erosion or 

 by subsidence, fur- 

 ther reduced the ca- 

 pacity of the streams 

 for cutting, and dur- 

 ing one epoch lime ooze (Niagaran) and during another mud 

 (Salina) accumulated on the sandstones and conglomerates. Where 

 the conglomerates have been tilted by later folding and cut by ero- 

 sion, their upturned edges form mountain ridges. 



The Silurian formations west of New York are largely limestone, 

 in portions of which well-developed coral reefs are to be distinguished. 

 The falls of Niagara are due to the presence of a massive layer of 

 limestone of Silurian Period, from which an important stage, the 



Fig. 411. — Map showing the probable outline of 

 North America during a portion of the Middle Silurian. 

 (Modified after Schuchert.) 



