H50 OBSERVATIONS ON 



a more plausible supposition ; but it would be altogether 

 gratuitous. Perhaps the Plutonists and Neptunists have l)oth 

 been wrong in refusing to admit of exceptions to their re- 

 spective theories ; and particularly with respect to the for- 

 mation of trap rocks, which probably ought not to be exclu- 

 sively referred to the agency of eitlier fire or water. 



But the Stony Ridge near Carlisle piesents ai)pearances 

 more decisively volcanic. In structure, the trap of wliich 

 its rocks are composed, differs but little from tiiat of the 

 Connewago Hills, except that it is somewhat harder, of a 

 finer graiuilation, and a darker colour ; but ifcis decomposed 

 in the same way, is covered by the same ferruginous coat, 

 and would, on being analysed, probably exhibit liut little dif- 

 ference of result in the relative quantity of its constituent 

 parts. The rocks differ more in size and shape, those of 

 the Stony Ilidge being smaller, and very rarely globular. 

 They present no columnar appearance, but at the Carlisle 

 Ironworks, where the ridge has been penetrated, I have ob- 

 served something like arrangement in their position, al- 

 though I cannot say they were of a crystalline form. It is 

 however not so much in the structure of its rocks, as in the 

 position of the ridse itself, that the evidence of its igneous 

 origin is found. The valley is here about twelve miles wide. 

 Its bottom is formed of an extremely compact transition 

 limestone, which, dipping at an angle of from thirty to forty 

 degrees, presents the broken edges of its strata, and forms 

 a pretty uneven surface. Between the Conodoquinnet creek 

 and the North, or as it is here called, the Blue Mountain, 

 the limestone is covered by schistus, and between the Yel- 

 low Breeches creek and the South Mountain by gravel. This 

 limestone formation, though it occupies the valley in nearly its 

 whole extent, certainly for a distance of five hundred miles, is 

 not exclusively confined to it, but appears at the same level to 

 the south of the South Mountain, and forms the soil of Frede- 

 rick county in Maryland, of a great part of Yoik, and of the 

 whole of Lancaster counties in Pennsylvania. The North and 

 South Mountains are composed of quartzose masses, of grey 



