2i8 GEORGE W. STOSE 



along the main axis in that direction. South of this point, how- 

 ever, by the yielding of the limestones of the valley in close folds, 

 the rocks of the mountains moved bodily to the west, past the end of 

 the anticHne, thus causing a deflection of the trend of the axis of 

 the main upHft behind this barrier, as may be observed by the change 

 in direction of ridges on opposite sides of Conococheague Creek. 



Earlier views. — The views of earlier writers on the geology of 

 South Mountain are concisely summarized by Keith in his paper 

 on the Catoctin belt.^ These opinions were somewhat diverse. The 

 two classes of rock composing the mountain were clearly distin- 

 guished by them all, and the igneous origin of the schists was gener- 

 ally recognized. The prevaihng conclusion of the earher writers, 

 however, was that the sandstones of the mountain front dip east- 

 ward beneath the schists, and are therefore older, and probably 

 Archean, and that a great fault exists along the west foot of the moun- 

 tain. These views were held as late as 1892 by Lesley, who constantly 

 refers in his final report^ to the '' lower or quartzite conglomerate slate 

 series" and "the overlying feldspathic, micaceous, chloritic series,' 

 thus reaffirming the interpretation of the stratigraphy and structure 

 published in earlier volumes. 



In this same year Walcott^ found fossils in the uppermost layers of 

 the sandstone series in the southeast corner of the area under dis- 

 cussion and in other parts of South Mountain, which, together 

 with stratigraphic observations cleared up the stratigraphy and 

 structure. The sandstone series was observed to overHe the ortho- 

 felsites and was proven to be of Georgian age. This view was 

 confirmed, on structural evidence, by Keith in his paper on the 

 Catoctin belt,-* although he found the structures in the area to the 

 south much more complicated by faulting than they are in this area. 



Lesley, in discussing the offsets in the mountain front, says:^ 



This geographical eschelon arrangement of the South Mountains is a good 

 indication of their geological structure It renders it probable that the strata, 

 whatever may be their age, have been thrown into a series of anticlinal and 

 synclinal waves entirely analogous to those with which the Paleozoic country 

 of middle Pennsylvania have made us so well acquainted. 



1 Loc. clt., pp. 318-20. 



2 Pennsylvania Geological Survey, Final Report, Vol. I, pp. 144-46. 



3 Loc. cit. p. 24. * Loc. cit., pp. 321-23. 5 Loc. cit., pp. 143-45. 



