I^AscoM.l AGE OF THE ROCKS. 27 
lulglom crates. In the South Mountain, in the absence of contemporaneous 
interbedded elastics, the occurrence is limited to the igneous rocks. 
The replacement of the feldspar by chlorite and epidote, and their 
I, replacement in turn by copper, characterizes the amygdaloids of the 
South Mountain also. 
The explanation of the precipitation of the copper is doubtless the 
same in both regions. 1 
A correlation of the South Mountain formations and the Keweena- 
^an copper- bearing rocks was suggested by Mr. Blandy in the article 
D jefore quoted. There is great petrographical similarity between the 
porphyrites and felsites and diabase porphyrites of the Keweenawnn 
series and their equivalents in the South Mountain, but it seems to the 
[ e writer unwise to parallelize on petrographical evidence the South 
Mountain igneous rocks and the Keweenawan of Lake Superior. All 
U jiorrelation upon such grounds, when applied to rocks widely separated, 
s well known to be untrustworthy. There has not as yet been found 
urfficient evidence to show to what horizon in the Algonkian series the 
t l jgneous rocks of South Mountain should be referred. 
tl 
COMPARATIVE AG-E OF SEDIMENTARY AND IGNEOUS ROCKS. 
CONTACTS DESCRIBED. 
That the Cambrian rocks do not underlie the " slates" and "ortho- 
li [elsites," as stated in the reports of the Pennsylvania surveys, is quite 
)lain, but whether the sediments are entirely subsequent to the 
gneous rocks or are in part, at least, contemporaneous, it is not so 
pasy to decide. 
Contacts between the sedimentary and igneous rocks are finely 
exposed at two localities. ' About halfway through the tunnel on the 
Gettysburg Uailroad the basic igneous rocks and the Cambrian rocks 
i Y lire in contact. Both formations dip gently to the southeast (±20°). 
|Che sandstone has become an indurated quartzite. Close to the green- 
„ g itone it has acquired a green color, due to the abundant development 
f chlorite. It has also become very schistose, and in fact might 
eadily be confused with the greenstone itself. Granulation, undu- 
atory extinction, and the obliteration of all direct evidence of clastic 
rigin, as revealed by the microscope, indicate the action of dynamic 
orce on the quartzite. The greenstone is slaty and too decayed at the 
01 1 tact for microscopic study. 
Southwest of Old Maria Furnace, in an abandoned cut on the Tape- 
worm Railroad, a great surface of red felsite 2 is partially exposed and 
artially overlain by the clastic formation. (PI. Yl.) Here again 
here is a gentle dip of both rocks to the southeast (i-15°). The sand- 
^umpelly, Geology of Michigan, Vol.1, Part III, p. 43. 
2 Felsito is used here as a field term for a compact, stony, nonporphyritic or inconspicuously por- 
lyritic volcanic rock. See pp. 37-38. 
