GRANITE-GNEISS IN CONNECTICUT 645 
that the granitic biotite-gneiss to the south, which has already 
been mentioned as constituting the lowest member of an 
anticlinal fold, and which appears from its field relations to be 
a sedimentary rock, wholly lacks the various characteristics of 
igneous origin possessed by the granite-gneiss. The miner- 
alogical and structural characters which come out in a micro- 
scopical study of the rock, and which will be described below, 
are also in harmony with the belief that the granite-gneiss, in 
spite of its general and pronounced foliation, is an igneous rock. 
II. PETROGRAPHY OF THE GRANITE-GNEISS 
1. Lhe ordinary type-—TVhe ordinary form of the granite- 
gneiss is a light-colored, rather fine-grained biotite-gneiss. The 
amount of biotite varies considerably. Where it is most abun- 
dant, the rock is a dark-gray, well-foliated gneiss ; where least 
abundant, the rock is light-gray and of a quite granitic appear- 
ance. In no case does it become perfectly massive. The grain 
of the rock allows of its being split along the foliation into curb- 
stones, but not, as in the case of some of the other gneisses of 
the region, into flagstones. In the main body of the granite- 
gneiss the strike of the foliation is approximately N. 45° W.; 
the dip, 30° N. E. Joint planes cut the granite-gneiss in sev- 
eral directions, one nearly or quite parallel to the foliation, others 
at right angles. The rock is handsome when first quarried, but 
has proved useless as a building stone, because it rusts on 
exposure. 
Microscopically the rock is a granitic mixture of feldspars 
and quartz, in which lie small plates of biotite. The biotite is 
abundant, generally with a pronounced parallel arrangement, to 
which the foliation of the rock is due. Hornblende is almost 
wholly absent. Ina single section green hornblende occurs in 
amount subordinate to the biotite and it can be seen in a very 
few exposures in the field. The feldspars usually present an 
allotriomorphic, granitic aggregate of grains. Orthoclase is most 
abundant. An acid plagioclase is of common _ occurrence. 
Microcline also occurs in smaller and more irregular grains. 
