16 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull^136. 
Many of the characteristic features of the South Mountain rocks were 
aptly described by Professor Rogers, yet, plainly, their nature was not 
fully understood nor their importance appreciated at the close of the 
First Geological Survey of Pennsylvania. In 18G0 the Primal series 
of Pennsylvania, as it occurs in Maryland, was thus characterized by 
Tyson: 1 
1. A hard sandstone, made up of grains of quartz, with occasionally grains of feld- 
spar and kaolin. 
2. A slate, varying in color from gray to brownish and greenish. It is ranked as 
an argillite, but portions of it assume a marked talcose appearance, especially in the 
Catoctin Mountain, and in parts of Middletown Valley, where it has been much dis- 
turbed and altered by proximity to intrusive rocks. These last consist of amphibo- 
lites (trap), porphyries, amygdaloid, serpentine, and epidote. This last-named rock 
is extensively developed, both in large masses and intercalated between the slates. 
The Second Geological Survey of Pennsylvania was organized in 1874 
under Prof. J. P. Lesley, who had been an assistant to Professor Rogers 
in the earlier survey. 
With improved facilities for scientific work, and with more accurate 
methods of mapping, a new survey of South Mountain was undertaken 
by Dr. Persifor Frazer, with A. E. Lehman as topographical assistant. 
Five sections (Nos. 7, 8, 9, 10, 11), more or less incomplete, were made 
through the mountain, and as the result of his investigations Dr. Frazer 
says : 2 
It is apparent that the great South Mountain is composed essentially of two 
groups of rocks, the lower (and along this line, the northwestern) consisting of 
various modifications of the quartz conglomerate above spoken of, and in which 
quartz occurs in various forms. 
The upper and southeasterly group is felsitic in character, but contains, also, large 
beds of hydromica and chlorite schists intersected by veins of milk quartz, while 
the orthofelsite presents every variety of appearance, from a sandy and earthy slate, 
in which the crystals of orthoclase are very much decomposed, indeed, are some- 
times almost clay, through the jasper-like variety to the massive and coarsely por- 
phyritic structure in which it is suited to be used as an ornamental building stone. 
In the same year (1877) Dr. Frazer published a further account 3 of 
the nature and origin of these "orthofelsites." This account was in 
substance twice repeated by him, in 1879 4 and in 1880 : 5 
The rocks of this region [South Mountain] may be divided into two great series — 
a western (underlying), of which the characteristic strata are composed of quartzite 
and of arenaceous schist containing quartz pebbles (Mountain Creek rock), and the 
eastern (overlying) of hydromica and chlorite schists, and orthofelsite, both porphy- 
ritic and unporphyritic. Both these series show indications of having been pene- 
trated by dikes of plutonic character within this area. 
The porphyry, which carries the copper in this region, shows no character of igneous 
action, hut occurs in coarse and thin beds, more or less disintegrated, and in certain 
'Report on the Geology of Maryland, Jan., 1860, pp. 34, 35. 
2 Report of Progress in the Counties of York, Adams, Cumberland, and Franklin, 1877, CC, pp 
285-295. 
3 The copper ores of Pennsylvania: Polytechnic Review, Nos. 16 and 17, Vol. Ill, April, 1877. 
-»Trans. Am. Inst. Min. Eng., Vol. VII, 1879, p. 338. 
6 Second Geol. Survey, Pa., CCC, Appendix, 1880, pp. 312-313. 
