bascom.] SUMMARY OF CONCLUSIONS. 83 
Hunt, as the quotations from these writers, given in Chapter I, show. 
While the igneous slates of South Mountain do not resemble any of 
the Cambrian sediments of that region, their resemblance to porphy- 
roids from regions where there has been extended lnetamorphism is 
very great. This resemblance is internal as well as external, and fur- 
nishes an instance of the production of essentially similar results by 
either of two different methods. 
The true nature of these slates and the manner of their production 
are conclusively revealed through held evidence. Where a single expo 
sure shows a shear zone of not more than 20 feet in which every gra- 
dation from a porphyry to a fissile slate is displayed, or where a single 
hand sx)ecimen shows such a metamorphism, the evidence of such a 
genetic relationship is irrefutable. 
Absoire of gradation between igneous and clastic rocks. — There is, on 
the other hand, no such gradation between the igneous rocks and 
undoubted elastics as might be expected if the former were metamor- 
phosed elastics. We have holocrystalline rocks sharply separated 
from noncrystalline elastics, with an entire absence of intermediate 
stages. 
Professor Kogers was impressed with the high degree of metamor- 
phism which these rocks must have undergone in order to attain their 
present holocrystalline character. "A gray siliceous altered rock," "a 
compact siliceous altered slate," are the terms he uses to describe the 
porphyries and aporhyolites, while he speaks of the porphyrites as 
"primal slate in a highly metaniorphic condition" and "highly altered 
greenish slate." The sediments and the igneous rocks have been sub- 
jected to the same dynamic forces, and, as a matter of fact, we find 
one no more highly metamorphosed than the other relatively to their 
respective powers of resisting alteration. 
Surface-flow features. — Positive field evidence for the non sedimen- 
tary origin of these rocks is found m the features which they possess in 
common with surface flows. Their vesicular, scoriaceous, and pumi- 
ceous character, the accompanying pyroclastics, their flow structures, 
even grain, conchoidal fracture, and other characteristics of a glassy 
lava all testify to an eruptive origin. 
PETROGRAPH1CAL EVIDENCE. 
Structural. — The petrographical evidence of the origin of the porphy- 
ries, aporhyolites, and porphyrites is of an even more unmistakable 
character. 
Their porphyritic structure is indicative of their origin. Olivine and 
feldspar phenocrysts with crystalline outlines, idiomorphic quartz with 
embayments and edges rounded by magmatic corrosion, are possible 
only in rocks which have once been molten. 
The ophitic structure, preserved in great perfection in the porphy- 
rites, is peculiar to rocks which have consolidated from a molten magma. 
