30 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
volcanics are not sufficient. In all probability there were several 
sources of lava flow. 
The southern vents furnished the great areas of basic lava (green- 
stones) in Maryland and southern Pennsylvania, while the northern 
vents poured out the enormous acid flows. 
In the Monterey district the two lavas are mingled, and apparently 
the basic flow was preceded by the acid flow. 
Thus the facts observed by Professor Williams in the north, Mr. 
Keith in the south, and the writer in the Monterey district, may be 
brought into accord. 
SUMMARY. 
Three types of rocks are present in the South Mountain : (1) Siliceous 
sedimentary rocks of Lower Cambrian age, overlying with structural 
conformity and stratigraphical unconformity igneous rocks. These 
igneous rocks are surface flows of (2) an acid and (3) a basic constitu- 
tion. They are probably pre-Oambrian, and lithologically resemble the 
Keweenawan copper-bearing rocks of Lake Superior. 
There is not sufficient evidence to decide their comparative age, 
though in the Monterey district field observations, on the whole, indi- 
cate that the acid rocks are the older. 
The subaerial flows were subjected to a limited erosion before the 
sediments were deposited and while they were being deposited, so that 
the region must have possessed some elevation at that time. 
The intense dynamic action shown by the igneous rocks occurred 
after the deposition of the sediments. Since the sediments were laid 
down the whole region has been subjected to lateral pressure (at the 
time of the Appalachian uplift), whereby the igneous rocks were cleaved 
and sheared and the sedimentary formation was thrust up over them 
from the east — where it has been largely eroded, occurring now only 
sporadically — and the whole region was elevated. That erosion has 
removed a great thickness of material since this elevation is indicated 
by the cleavage dips of the igneous rocks. 
