GRANITE. 125 
Granite—In the third volume of the Geology of Wisconsin, I have 
spoken of granite as occurring in intersecting masses in the coarse gabbros 
which form the base of the Keweenaw Series in that region, as also in the 
immediately underlying mica-schists of the Huronian; and a number of 
these granitic areas were mapped on the accompanying atlas plates. This 
granite’ is there described as a red biotite-granite, with orthoclase and 
liquid-bearing quartz as the chief constituents, and with mica, rarer white 
plagioclase and still rarer apatite and magnetite, as subordinate constitu- 
ents. For the sections examined this description is entirely correct; but 
most of these few sections were from the granites of the underlying Huron- 
ian. For the present memoir I have had a number of new sections cut, 
and find that the granites cutting the Keweenawan gabbro are more often 
hornblendic than micaceous, and that all of these granites, including those 
of the underlying Huronian, are very closely allied to the coarse-grained 
granitoid rocks of the immediately preceding tables, from which they merit 
separation only from their relatively large content of the hornblendie or 
micaceous ingredient. The hornblende of these rocks is of the basaltic 
variety, with an intensely deep-brown color and very strong dichroism. 
Here and there an augite core is to be noticed in the hornblende, which is 
thus plainly a secondary product. It is always in regularly outlined crys- 
tals. The feldspars are always much reddened and altered, and were evi- 
dently much shattered and corroded before or during the deposition of the 
quartz. The latter ingredient is very abundant, crowded with liquid-filled 
cavities, many of which carry a minute salt cube, and while not certainly 
a secondary product, was subsequent, as just stated, to much breaking and 
honeycombing of the feldspars. Besides the very coarse and more abund- 
ant quartz areas, there are in some sections aggregates of numerous little 
particles between the coarser constituents, and it is mingled with these 
finer quartz particles that the minute biotite flakes are found in those sec- 
tions carrying this mineral. 
Although, as already said, so closely allied to the augite-syenites and 
augite-granites, or granitells, of the preceding tables, rocks so completely 
granitic in character as these have not been observed elsewhere in the 
Keweenaw Series. 
1 Op. cit., p. 193. 
