476 C. D. Walcott — Cambrian Bocks of Pennsylvania. 



Adams counties, form the north era most end of the Blue Ridge 



range of Virginia The whole measures upon the 



map ten miles in breadth by fifty in length upon a curve ex- 

 tending from the Maryland line to its eastern edge, fifteen 

 miles west of Harrisburg.* 



From the Pennsylvania line southwest across Maryland, 

 South Mountain extends, as the Blue Ridge, to Harper's Ferry, 

 and thence southwest across Virginia. It, also, practically 

 includes the Cotoctin range, on the eastern side, which extends 

 south from the southwestern portion of Adams county, Penn- 

 sylvania, and crosses the Potomac at the Point of Rocks, and 

 from thence extends south a little west of Leesburgh, Va. 

 The Blue Ridge and the Cotoctin Ridge are the eastern and 

 western sides of the mountain uplift of which the South Moun- 

 tain, Pennsylvania, is the northern terminus. 



The classification of the rocks of Pennsylvania was summed 

 up by Prof. H. D. Rogers as follows : — The Hypozoic rocks, 

 or those underneath any life-bearing strata ; Azoic, or those ' 

 destitute of any discovered relics of life ; and Paleozoic, or 

 those entombing the remains of the earth's most extinct forms 

 once living beings.f 



It is evident from Prof. Rogers' definition of the Azoic 

 group, that it included what we now recognize as the lower 

 Cambrian sedimentary strata beneath the Scolithus quartzite 

 and, also an extended- series of altered rocks that form the 

 nucleus of the Blue Ridge, and which are now included in the 

 Algonkian of the classification of the U. S. Geological Survey. 

 He regarded the sandstone with Scolithus linearis as at the 

 base of the Paleozoic series, and considered that the Primal 

 slates beueath the sandstone, and in intimate alternation with 

 it, did not possess a vestige of organic life. 



The conclusions of the geologists of the second geological 

 survey of Pennsylvania, are that there are two groups of rocks 

 forming South Mountain. 



Prof. Lesley says : " The northwestern (Mt. Holly) ridge is 

 made by several thousand feet of the lower quartzite and quartz 

 conglomerate beds. The southeastern (Adams county) ridges 

 are made by several thousand feet of an overlying feldspathic, 

 micaceous and chlorite series, intersected by veins of milky 

 quartz." J .... " It is hard to avoid the inference that our 

 South Mountain rocks represent the Huronian section of Murray 

 and Logan. It is impossible not to compare them also with 



* Second Geol. Survey of Penn. A summary description of the geology of 

 Penn., vol. i, 1892, p. 142. 



fThe Geology of Pennsylvania, vol. i, 1858, p. 64. 



% Second Geol. Surv. of Penn A summary description of the geology of Penn- 

 sylvania, vol. i, 1892, p. 144. 



