' 
28 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136 
stone has become a vitreous quartzite. The hard felsite does not sho 1 
alteration. That the igneous and sedimentary rocks have been sul 
jected to the same forces of folding is shown by the uniformity of then 
cleavage dips at both these localities. 
This conformity of structure planes, the absence of contact meta- 
morphism, and the evidences of dynamic action indicate planes of 
thrust rather than planes of original deposition or of subsequenl 
igneous intrusion. 
CONCLUSIONS. 
These two contacts do not decisively prove the younger age of t 
overlying sediments, although they very strongly indicate it. If th 
sandstone has been thrust over the lava from beneath, it would be ne 
essary to suppose an enormous amount of erosion to account for th 
entire absence of volcanic material above the sandstone. On the other 
hand, it is very easy to suppose that overlying sediments, on being 
subjected to pressure, were thrust up over the igneous rocks from the 
east. This explanation of the facts coincides with other evidence rel 
tive to the comparative age of the sedimentary and volcanic rocks. 
(1) Nowliere in the South Mountain has there been found a dike of 
the volcanic material in the sedimentary rock. Were the lavas more 
recent than the sediments, the former could hardly fail of tilling cracks 
in the latter. 
(2) No sedimentary beds have been found intercalated with the lava 
flows. 
(3) There is no evidence of the alteration of the sedimentary rock 
by igneous contact. 
(4) The igneous rocks, in their fluxion and amygdaloidal structures, 
and in their accompanying pyroelastics and ash deposits, bear every 
evidence of being subaerial lava flows. 
(5) The presence of igneous fragments in the basal conglomerate of 
the sedimentary formation shows that erosion of the igneous rocks was 
taking place while the sediments were accumulating along their edges. 
While it is true that some erosion of the eruptives must have taken 
place before and during the deposition of the Cambrian sediments, 
there is no evidence of extended erosion; indeed, the character of sur- 
face flows, which the lavas retain so conspicuously, would incline one to 
suppose that no great load of material has been removed. Mr. Keith's 
conception that the granite which in Maryland is intruded into the 
"diabase" (greenstone) is younger than the volcanics necessitates 
an immense original thickness of the diabase flows and an extended 
subsequent erosion to expose the plutonic eruptive. In this respect 
his argument for the comparative age of the deep-seated and surface 
igneous rocks seems at fault 1 . Such unlimited erosion is unsupported 
by field observations in the South Mountain. 
1 Geologic structure of Blue Ridge iu Maryland aud Virginia: Am. Geologist, Vol. X, 1892, p. 368. 
