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ACID ORIGINAL ROCKS. 91 
than the lower zone: in them the tendency is undoubtedly towards the formation of 
quartz, chlorite, and epidote rocks as a more stable limit, through the mediation of 
prehnite and calcite. 
There are other forms of alteration which Pumpelly’s investigation 
does not cover, but none of so great importance as those above deseribed. 
SEcTIon IIl.—ACID ORIGINAL ROCKS. 
As indicated in a previous chapter, I have been able to show that the 
several kinds of felsite and acid porphyry, which make the pebbles of the 
conglomerates and the material of most of the sandstones of the Keweenaw 
Series, exist in the same series in the original condition; and that while sub- 
ordinated to the basic rocks in total amount they yet form a very important 
element in the make-up of the series, throughout its entire circuit about 
the Lake Superior Basin. These acid rocks may be conveniently described 
under the following heads: 
1. Quartzless porphyry. 
2. Quartziferous porphyry and felsite. 
3. Augite-syenite, and granitell or granitic porphyry. 
4, Granite. 
Quartaless porphyry.—There are several phases of porphyritic rocks in 
the Lake Superior region, occurring both as pebbles in the conglomerates of 
the Keweenaw Series and as flows in the same series, which would form- 
erly have been classed together as “quartzless porphyries,” that name 
applying to felsitic rocks in which quartz is present neither in the base nor 
as a porphyritic ingredient." These several phases have in common an 
aphanitic, dark-brown base, frequent abundance of porphyritic feldspars— 
although kinds occur in which the feldspars sink out of sight—and freedom 
from visible porphyritic quartz. They are also distinctly softer than the 
true acid quartziferous porphyries. A study of the thin sections, however, 
aided by silica determinations, has shown that in such a grouping we 
should really be placing together kinds which are but the half crystalline 
or cryptocrystalline phase of the less basic diabases, others which verge on 
‘Conf. R. Pumpelly in Geological Survey of Michigan, Vol. I, Part II, p. 16. 
