Tlic Rocks of Blue Mountain. 85 



valley. On the western or Blue Ridge side it is built u}) of sedi- 

 mentary rocks originally deposited in the sea on the bottom and, 

 it may be, the side of the Appalachian trough. In the interven- 

 ing valley it consists to a considerable extent of eruptive rocks, 

 which poured out as flows the ancient land surface prior to the 

 existence of the Appalachian trough and before the deposition 

 of the stratified rocks which so largely form the North American 

 continent within the limits of the United States. The elevated 

 eastern side forms the Catoctin ridge, which is capped by a com- 

 pressed fold of the old shales and quartzites. Both ridges con- 

 tinue south of the Maryland line toAvard Harpers Ferry and far 

 into Virginia as compressed synclinal folds of the Cambrian 

 rocks, resting on the rocks of the ancient Appalachian trough, 

 the older rocks and the more recent rocks having been involved 

 in the same series of folding. In addition to this folding, numer- 

 ous thrusts of one mass of rocks upon another are to be found 

 all along the Blue ridge, especially north of the Pennsylvania- 

 Maryland line, in the northern extension of Blue mountain, 

 or the South mountain of Pennsylvania. In some instances the 

 ancient eruptive rocks have been thrust westward, so as to rest 

 upon and above the more recent sandstones and shales which 

 were originally deposited upon them in the bottom and along 

 the shore of the Appalachian trough. Often the pressure has 

 cleaved the massive lavas and formed slates and shales that 

 appear like those deposited in quiet waters. The result of this 

 has been to complicate the geologic structure and topography of 

 South mountain and the Blue ridge, and to make the region 

 one of great interest to both professional and amateur geolo- 

 gists. Erosion has aided their study by cutting away thousands 

 of feet of strata from above the present mountain area and adja- 

 cent valleys, and thus laying bare a portion of the ancient shore- 

 line of the Atlantic coast area of Cambrian time and of the 

 foundation upon which much of the present continent is built. 

 The history of the Blue ridge and its rocks as now interpreted 

 is essentially as follows : '^ It began long after the first known 

 primitive rocks of the earth were raised into plateaus and ridges 

 to form the platforms of the present continents. At the close of 

 the periods in which the earlier crystalline rocks of the conti- 

 nent were formed, and also the great masses of bedded rocks 

 beneath those containing the Cambrian or oldest known fauna, 



*See Am. Journ. ScL, vol. xliv, 1892, pp. — . 



