386 THE SILURIAN PERIOD 



and western New England, into permanent land. The Appalach- 

 ian land was enlarged and connected with the northern area, shut- 

 ting off the straits which had formerly separated the two areas and 

 had joined the Interior Sea with the Gulf of St. Lawrence. South- 

 ern Ohio and central Kentucky and Tennessee had been uplifted 

 into one or more considerable islands, which shut off the north- 

 eastern portion of the Interior Sea as a partially closed gulf, while 

 another island occupied southern Missouri. What changes affected 

 the land masses of the far West and Southwest cannot yet be defi- 

 nitely determined, but it is probable that they were enlarged. 



The Silurian rocks are much better developed and far thicker 

 in the East, especially along the Appalachian range, where they 

 attain great thickness, than in the interior or western regions, for 

 they thin out or are wanting over large parts of the latter. So far 

 as they have been identified, these rocks were laid down in the 

 Interior Sea or in the narrow channels which still extended from 

 the Gulf of St. Lawrence southward into New England. 



The period opened with the formation in the East of a thick 

 conglomerate of quartz pebbles and sand, the Oneida (a substage 

 of the Medina). This conglomerate is extensive in central New 

 York, thinning toward the eastern shore line, but very thick along 

 the Appalachian ranges as far south as Tennessee. Owing to its 

 hardness and resistant qualities, it forms the crest of many of the 

 mountain ridges, as of the Kittatinny, at the Delaware Water 

 Gap. Farther west and overlying the conglomerate is a great mass 

 of sandstone, the Medina, which was accumulated in very shallow 

 water, for it is abundantly ripple-marked. The sandstone extends 

 westward from central New York, thinning out to a belt of shale 

 in Ohio ; reduced to a thickness of 300 to 400 feet in Ontario, it 

 does not reach Michigan, but is very thick in eastern Pennsyl- 

 vania (1800 feet). No equivalent of these beds is known in the 

 interior. 



In New York and Pennsylvania the sandstone is followed by a 

 series of shales and shaly sandstones with some beds of limestone, 

 the Clinton, which is much more widely extended, reaching south- 

 ward along the Appalachians to Georgia, and westward as far as 



