CD. Walcott — Cambrian Rocks of Pennsylvania. 479 



section, however, there is in addition a series of shales beneath 

 the Scolithus quartzite. that rests upon a massive quartzite 

 forming the summit of the Blue Ridge, west of Monterey and 

 beneath this a bed of slates unconformable to the subjacent 

 crystalline rocks. 



The Blue Ridge was followed south into Maryland and 

 crossed at several points before reaching Harper's Ferry. All 

 of the section shows the synclinal structure of the slates and 

 quartzites as represented by Messrs. Geiger and Keith, in their 

 paper upon the structure of the Blue Ridge near Harper's 

 Ferry.* South of Keedysville, Washington county, Mary- 

 land, the quartzite, capping the slate hills west of the main 

 ridge, was observed to pass conformably beneath the limestone 

 at Eakle's Mills, and Hyolithes communis and fragments of 

 Olenellus were found in the calcareous quartzite. The rela- 

 tively simple stratigraphic structure of the Monterey section 

 is complicated at, and near, Harper's Ferry by the lower 

 massive quartzite forming a synclinal and being thrust to the 

 westward over the more recent shales, slates and limestones. 

 The structure is still more complicated by the fact that the 

 hills of sandy shale and slate (capped by the upper Olenellus 

 quartzite) are thrust, on the line of a fault, over on to lime- 

 stones which, in an unbroken section, rest upon the quartzites. 



It was this primary folding and subsequent westward thrust- 

 ing, on the line of two or more faults, of the older upon the 

 more recent strata at and to the north and south of Harper's 

 Ferry that led Messrs. Geiger and Keith to consider that the 

 lower quartzites rested conformably upon the limestones and 

 were of Silurian age.f 



Returning to South Mountain with the information gained 

 between the Potomac and the line of the Chambersburgh and 

 Gettysburgh pike, in Pennsylvania, and studying Dr. Frazer's 

 sections (Nos. 7, 8, 9, 10, 11 and 13)^ and also reading the 

 descriptions of them, as well as Professor Lesley's description 

 of South Mountain (contained in Yol. I of his final report), 

 it is evident that they have misinterpreted the true geologic 

 structure of the mountain and the relations of the rocks com- 

 posing it. Professor Lesley states that a massive fault must 

 run along the foot of the mountain, along the low drift-filled 

 valley of Yellow Breeches creek ; and this I think is correct, 

 as the Olenellus fauna of the Scolithus quartzite zone occurs 

 but a short distance east of the foot of the mountain, in a syn- 



*Bull. Geol. Soc. America, vol. ii, 1891, pis. 4 and 5. 



f Loc. cit, pis. 4 and 5. A paper by Mr. Arthur Keith describing his present 

 view of the structure will be found in the Dec. No. of the Am. Geologist for 1892. 



% Second Geol. Surv. Pa. Report of Progress in the counties of York, Adams, 

 Cumberland and Franklin for 1875 published 1877. 



Am. Jour. Sci. — Third Series, Vol. XLIV, No. 264. — December, 1892. 

 32 



