62 
VOLCANIC ROCKS OF SOUTH MOUNTAIN. 
BULL. 136. 
ses, and is it convincing proof of the igneous origin of the rocks which 
they represent. In the absence of samples of the rocks analyzed or of 
exact descriptions of their character, special points can be brought out 
only inferentially. 
The high percentage of the alkalies and the slight trace of lime 
plainly denote the character of the feldspathic constituents. This 
indication of their chemical character coincides with the optical and 
physical properties enumerated on page 40. The rock from which 
Analysis IV was made is reixu'ted to be from the same locality from 
which many of the aporhyolites were obtained, the optical character of 
whose feldspars were tested. 
Microscopic study of these rocks leads us to expect a percentage of 
titanium oxide. In two instances the analyses show it. It is not 
unlikely that in the other cases the titanium oxide was not determined. 
The absence of manganese oxide from these analyses is surprising, as 
the Monterey porphyries and aporhyolites show a high percentage of 
it. The lime and magnesia present are doubtless due to the presence 
of epidote in the rock. The silica, alumina, and iron percentages are 
exactly normal and call for no remark. 
Analysis I, of a fissile schist, illustrates the slight change of chemical 
constitution which accompanies dynamic action in the acid rocks. 
Among the analyses made by Dr. Genth for the Second Geological 
Survey of Pennsylvania are the following, which, because of their 
anomalous character, have not been tabulated with the others : 
I. "Slaty rock." " Nine miles southwest of Dillersburg." 
II. "Purplish slaty orthofelsite." "One and one-half miles southeast of Mount 
Alto." 
These rocks plainly do not represent the normal type of the South 
Mountain acid rock. In the absence of specimens or means of deter- 
mining the character of the rocks from which these analyses were made, 
it is impossible to explain altogether satisfactorily the abnormally low 
percentage of silica and the high percentages of alumina and iron. 
If these slates were once normal aporhyolites, the shearing which 
produced the subsequent slaty character must have been accompanied 
by an abundant development of sericite from the feldspar. If the silica 
