?0 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
There are a few other localities where slates occur. Where the slates 
were not studied in thin section their igneous origin has not been 
considered as proved. 
Ash beds, diabases crashed and recemented with epidote and quartz, 
are exposed along the Gettysburg Railroad in the cats west of the 
tun neh 
A tuffaceous breccia, composed of fragments so rounded as to appear 
water worn, was found at the head of Mime Branch. At the Russel cop- 
per mine and a few other localities epidosites are abundant. At the 
former place they are evidently vein material, and carry the native 
copper. In other localities they undoubtedly represent the last stages 
of decomposition and alteration of the massive or more often of the 
tuffaceous diabases. 
MACKOSCOPICAL DESCRIPTION. 
The augite-porphyrites and melaphyres vary in color from a slate- 
blue or purple to all shades and tones of green. Where epidote is the 
predominating alteration mineral the prevailing color is light yellowish- 
green 5 with chlorite or actinolite as the alteration products the color is 
a dark green. 
The most persistent and striking feature of the augite-porphyrites 
and melaphyres is their amygdaloidal character, to which allusion has 
already been made. 
Bowlders on the roadside and in the fields show a curiously rough 
and pitted surface, due to epidote or quartz amygdules brought out in 
relief by weathering. Sometimes the bowlders closely resemble con- 
glomerates composed of green or white oval pebbles; or the quartz 
amygdules, when perfectly spherical, mimic the spherulites of the 
acid rocks. 
The diabases (augite-porphyrites and melaphyres) are rarely mas- 
sive, usually schistose, sometimes slaty, and almost universally amyg- 
daloidal. 
As the amygdules quickly respond to pressure, they furnish a deli- 
cate test of the degree of schistosity present in the rock. Macroscopic- 
ally the schistosity is otherwise more or less obliterated by subsequent 
epidotization or chloritization. 
Genuinely massive diabases are exceptional in occurrence and lim- 
ited in extent. Occasionally a mass of rock has moved as a whole, 
under pressure, and thus close to schistose or even slaty diabase the 
rock may retain its massive character. In these cases, since there has 
been no shearing, the vesicles are often perfect spheres, showing no 
elongation from inagmatic movement. This is notably the case just 
north of Gum Spring, on the Old Furnace Road. Quartz amygdules 
show conspicuously as round white spots on the fresh surface of the 
rock, which is a dark blue-gray, or are brought out in relief on the 
weathered surface, giving the rock the appearance of being riddled 
