480 C. D. Walcott — Cambrian Rocks of Pennsylvania. 



clinal fold, at Mt. Holly Springs. Their error, however, is in 

 considering that the " orthofelsite " series is superior to the 

 conglomerates, quartzites and schists which they referred to 

 the Lower series. The Monterey section shows that the epi- 

 dotic schists are inferior to the quartzites and slates and, a 

 section west of Wolfsville, Md., that the " petrosilex "• or rhyo- 

 lite-like eruptive occupies a similar position. This type of 

 section is repeated many times, both on the Cotoctin and Blue 

 Ridge sides, from the Maryland line to the Potomac and south 

 through Virginia. 



Professor Rogers and also Professor Lesley, referred the 

 offsets of the ranges of hills of South Mountain, as shown in 

 Franklin county and also on the north end of South Mountain, 

 to the terminations of successive folds of the rocks forming 

 the mountain. My impression is that these offsets and also the 

 complicated structure of the mountain arise partly from fold- 

 ing, but more largely from the westward thrusts of masses of 

 strata along the line of faults of a low hade. This westward 

 thrusting on the fault planes, complicated by previous foldings 

 of strata, leaves masses of the subjacent pre-Paleozoic rocks 

 resting, in various places, on different members of the lower 

 Cambrian series, and also appears to interbed the quartzites 

 and slates of the Cambrian in the schists, eruptives, etc., of 

 the Algonkian.* 



The key to the succession of the lower sedimentary rocks of 

 Maryland and Pennsylvania is contained in the Balcony Falls 

 section of Virginia, although it can now be determined by a 

 study of the section at Monterey and to the south, along the 

 Blue Ridge toward Harper's Ferry. 



In a letter received from Professor Lesley and dated Feb- 

 ruary 22d, 1891, he asks : " Is it impossible that there should 

 be agreement between the Balcony Falls section of Virginia 

 and the Mt. Holly Springs section, three hundred miles apart ?" 

 He says, further, after commenting upon the possible relations 

 between the Balcony Falls section and that at South Mountain, 

 in speaking of the strata of the Balcony Falls section : " But 

 what is 2,000 feet or 2,500 feet to 10,000 feet to 20,000 feet of 

 quartzites and slates making (apparently — not certainly — ) the 

 South Mountains? We are still in the dark about super- and 

 swJ-positions ; about absence or presence of overturn rolls, etc. 

 I am only greatly impressed with the broad fact that we seem 



* From the finding of fragments of the eruptive rocks in the conglomerates at 

 the base of the quartzite series, and from the numerous synclinals showing that 

 the epidotic rocks and also certain rhyolitic eruptives are beneath the quartzite 

 series I refer the similar rocks of South Mountain to a pre-Paleozoic age ; and, 

 as they are not of the character of the Laurentian crystalline complex, I would 

 refer them to the Algonkian. but not correlate them with the Huronian or with 

 any known division of that group of rocks. 



