CHAPTER VI. 
SUMMARY OF CONCLUSIONS. 
EVIDENCE OF THE ERUPTIVE CHARACTER OF THE TWO ROCKS. 
The preceding chapters have treated in detail structural and chemical 
features possessed by the South Mountain rocks and characteristic of 
igneous rocks only. These are regarded as sufficient evidence of the 
igneous origin of the rocks which they characterize, without further 
proof on that point. Jt only remains to show on what grounds a sedi- 
mentary origin has been attributed to them, and to sum up the evidence 
against such an origin. 
FIELD EVIDENCE. 
Schistosity. — The conformity of the foliation planes of the porphyries, 
the aporhyolites, and the porphyrites, with the foliation planes of the 
Cambrian sediments is a prominent and j>ersistent feature, and one 
which, among others, has undoubtedly led to the ascription of a sedi- 
mentary origin to the former rocks. The confusion of foliation planes 
and bedding planes in the quartzite, owing to the obscurity of the 
latter, added force to this argument. 
This conformable schistosity is not, of course, inconsistent with the 
igneous origin of the underlying rocks. The schistosity is a secondary 
feature, produced by forces which affected the igneous and aqueous 
rocks alike. 
The cleavage is quite as plainly secondary in the clastic rocks as in 
the nonclastic, and sometimes conforms to the bedding and sometimes 
does not. 
Lamination. — An original lamination, conspicuous in the aporhyo- 
lites, the nature of which was described on pages 43-44, characterized 
as bedding by Hunt and others, doubtless furnished another reason for 
attributing stratification and an aqueous origin to the rocks possessing 
it. The real nature of the lamination has proved to be such that it 
becomes an evidence of the igneous character of the rocks in which it 
occurs. 
That the lamination is due to bands of spherulites has been pointed 
out. True spherulitic crystallization, such as has been described in 
these aporhyolites, has thus far been known only as the product of crys- 
tallization from a molten magma. 
The slates. — The slaty character of the rocks has been another reason 
for assigning a sedimentary origin to them. 
The slates, both acid and basic, have been considered clastic slates 
by Professors Rogers and Lesley, by Frazer, Tyson, Blandy, and Dr. 
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