24 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull.136. 
Economic value. — The susceptibility of these porphyries to a fine 
polish and their suitability for ornamental purposes have been remarked 
by many of those who have studied them. 
The earliest mention of this sort was made in 1822 by Dr. Hayden, 1 
who notes " handsome porphyry, Nicholson's Gap, Blue Ridge, Penn- 
sylvania, crystals red and distinct." 
Speaking of Oatoctin Mountain, the southern extremity of South 
Mountain in Maryland, Tyson says: 2 
Its porphyries and amygdaloids are deserving of the attention that I propose here- 
after to bestow upon them. Some of them will receive a beautiful polish, bat their 
hardness renders the process expensive. This can, however, be overcome by appro- 
priate machinery. 
Dr. Frazer 3 has, as already quoted, described the porphyries as 
" suitable for an ornamental building stone." 
Dr. Hunt 4 has also called attention to the fact that " these peculiar 
rocks, which make such a conspicuous figure in the South Mountain of 
Pennsylvania south of the Susquehanna, are of interest economically 
from the fact that they are in other regions the repositories of rich 
iron ores, and also because they afford ornamental porphyries of rare 
beauty, similar to those wrought in Elfdalen, in Sweden." 
The Tenth Census 5 reports that "it [the South Mountain porphyry] 
is well adapted to ornamental work, as it is rich in color, durable, and 
susceptible of a good polish, and in many cases could be obtained in 
abundant quantities. It has not yet been quarried for purposes of 
construction." 
BASIC ERUPTIVES. 
Areal distribution. — The basic eruptives occupy, in this district, an 
area fully twice as large as that covered by the acid eruptives consti- 
tuting the major part of the valleys, foothills, and mountain flanks. 
Character. — The rocks are massive, schistose, or slaty. They are 
usually conspicuously amygdaloidal, and associated with these amygda- 
loids are banded fine-grained schists, which have been considered altered 
accumulations of volcanic ash. In a section exposed on the Gettysburg 
Railroad there are alternating bands, from 2 to 3 feet wide, of a compact, 
fine-grained, epidotic rock which may also represent a basic volcanic 
ash. Bombs were found embedded in this epidotic rock. As with the 
acid rocks, there are accompanying basic breccias, though the latter are 
not abundant. The cementing material in every case was epidotic. 
An agglomerate formed of rounded fragments from an inch to 6 inches 
in diameter was also found. There was no opportunity for estimating 
the thickness of the accumulated basic flows. Wells have been bored 
in the basic rock to the depths of 55, 85, and 110 feet. 
1 H. H. Hayden, Mineralogical notes : Am. Jour. Sci. and Arts, Vol. V. 1822, p. 255. 
2 Tyson, First Annual Report, 1860, Appendix, p. 3. 
3 Frazer, Vol. CC, p. 285. 
4 Hunt, Proc. Am. Ass. Adv. Sci., 1876, p. 212. 
6 Tenth Census Report on the Building Stones of tbe United States, p. 168. 
