bascom.] HISTORICAL REVIEW. 15 
The geological structure or mode of stratification of this belt is equally simple. 
The whole tract consists of two or three groups of high, narrow, nearly parallel 
anticlinal ridges, expanding and subdividing toward the southwest. These are 
composed of the Primal white sandstone. Between them are high parallel valleys 
and plateaus of the Primal upper slates, which, from being softer and more fissile, 
have been worn and trenched by the plowing force of waters to somewhat lower 
levels than the more resisting, . ter cemented sandstones. The crests of the ridges 
are therefore stony and rugged, their flanks usually smoother, being formed chiefly 
of the slate. 1 
That Professor Rogers did not include in the " singularly small 
amount" of igneous rocks the rocks forming the valleys, the mountain 
flanks, and even the summits of the mountains, is further indicated by 
the following: 
Another section across the mountain more to the southwest extends from south to 
north along the Baltimore and Carlyle turnpike. The first important stratum of the 
hills is the usual gray siliceous altered rock, so common along their southern side. 
North of this, about 3 miles from Petersburg, occurs the dark green slate, with its 
epidote and white intrusive quartz. Succeeding this is an extremely compact sili- 
ceous altered slate, and beyond this a reddish gray rock, of the same series, containing 
specks of reddish feldspar and small veins of epidote, and near this the fissile tal- 
cose rock, several times mentioned before. * The summit of the ridge exhibits 
a dark blue and greenish blue indurated rock, weathering a dark brown, and evi- 
dently very ferruginous. It appears to be a band of the Primal slate in a highly met- 
amorphic condition, approaching jasper. * * * These lower Primal slates are 
highly indurated, and even decidedly crystalline, containing in some of their layers 
segregated specks, and even half-formed geodes of epidote and other minerals. 
On page 206 of the same report Professor Eogers has figured a sec- 
tion across South Mountain, along the Gettysburg Railroad, from Fair- 
field to Monterey Springs. This section shows stratified rocks lying 
in a series of anticlinal flexures, which accord rather with Professor 
Rogers's conception of "rock waves" than with his observed dips. 
These dips are, with a single exception, to the southeast. Accompany- 
ing the section is the following description, which bears directly upon 
the portion of South Mountain that is particularly discussed in this 
bulletin : 
Passing now to the eastward of the Green Ridge axis we cross a high slope of 
slate, apparently the upper Primal, in a synclinal fold, and then traverse a succes- 
sion of outcrops of the Primal white sandstones and slates to the eastern base of the 
high land called Jack's Mountain, at the foot of which the older rocks disappear 
under the Mesozoic red sandstone of the plain of Adams County. 
The exposures in the sandstone near the tunnel opposite Jack's Mountain indicate 
a probable thickness of 1.000 feet. Near the tunnel at the northwest side of the 
mountain there is a hard epidotic rock, and not far from it a highly altered greenish 
slate, a rock found in several other localities farther west and containing layers of 
gray slate, spotted with epidote. Farther west occurs epidote with asbestus. Near 
Mmie Branch search was made many years ago for copper ore, but nothing was found 
to justify the expectation of finding a productive vein of that mineral. A small 
quantity of copper ore was once obtained and a furnace built for smelting it in a 
small ridge north of Jack's Mountain, but the exploration was abandoned. The 
metal occurs in the form of a green and blue carbonate, with a little native copper. 
Evidently the ore is not abundant. 
Tlio italics in this and tli«' following quotations are tho writer's. 
