80 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
Summit station, the roadway crossing the Gettysburg- Railroad in front 
of the Clermont House, and the hill to the east of the Fairfield-Foun tain- 
dale road and north of the Fountaindale turnpike, are the other locali- 
ties where slates occur. While their association and their appearance in 
the hand specimen, which resembles that of the first two slates discussed, 
indicate an igneous origin, in the absence of thin sections that origin 
can not be considered beyond question. 
BASIC PYROCLASTIOS. 
DISTRIBUTION AND DESCRIPTION. 
The basic breccias of South Mountain may be classified as follows : 
(1) Crushed porphyrites which have been recemented with epidote and 
quartz and sometimes brilliantly colored with red hematite; (2) Tuffa- 
ceous breccia j (3) Ash. 
(1 ) Crushed porphyrites. — The crushed and broken porphyrites, while 
perhaps not in a strict sense breccias, present a strikingly brecciated 
appearance. Fragments of all sizes, of a blue or purple-gray rock, are 
embedded in a bright-green and white or rose-colored matrix. The 
fragments have undergone either silicification or epidotization without 
affecting materially their structure (microophitic), which is outlined 
by iron oxide. 
(2) Tuffaceous breccia. — Some large bowlders found near the source 
of Minie Branch furnish the only unmistakable tuffaceous breccia. 
The fragments show a considerable range in size (see page 24) and are 
thickly crowded in a basic cement. As there has been no shearing, 
the structure of the fragments is perfectly preserved. (PI. XXVIII, a.) 
Epidote, quartz, and iron oxide are their present constituents. 
(3) Ash. — Above the Headlight copper mine on the Fountaindale 
turnpike and in the fourth cut beyond Monterey station (northeast), on 
the Gettysburg Railroad, are intercalated bands of a light-green rock 
w 7 hich, for the following reasons, have been considered altered ash: 
At the west end of this cut a fine-grained homogeneous rock is striped 
with alternating bands of light green and reddish green. Under the 
microscope these bands show no trace of any structure save a slight 
schistosity. They are composed almost wholly of angular grains of 
epidote, magnetite and leucoxene, actinolite, chlorite, and quartz. The 
difference in color is due to the presence in the reddish bands of red 
iron oxide. Toward the eastern end of the same cut the whole face of 
the rock is banded with light-green epidotic layers from 1 foot to 2 
feet wide, running approximately parallel to one another. 
A microscopic slide of one of these bands consists wholly of granular 
epidote and quartz, with a little iron oxide, usually the red oxide. 
These rocks overlie and are in close proximity to scoriaceous basic 
lava. This fact, together with their variation in color and their struc- 
tureless and fragmental character, is very suggestive of an altered ash. 
At the first locality mentioned, the Headlight copper mine, the so- 
