352 HENRY B. KUMMEL 
their distribution on geologic maps is quite impracticable. ‘The 
most noteworthy differences of appearance presented by them are 
those of color, and inasmuch as color-distinctions have been found to 
correspond broadly with fairly definite lithologic differences, they 
may be used as a guide in classifying the gneisses for the purposes of 
description and mapping. 
All the dark gneisses which owe their color to the hornblende, 
pyroxene, or biotite which they contain have been grouped together 
under the name Pochuck gneiss. ‘The second group, the members 
of which show brown-gray, bronzy, pink, and ocherous tones, is called 
the Byram gneiss. Here are included a great variety of granitoid 
or granite-like rocks related to one another and distinguished from 
the other gneisses by the presence of potash feldspar as an essential 
ingredient. A third group, the Losee gneiss, includes light-colored 
granitoid rocks, many of them nearly white, which contain lime-soda 
feldspar as an essential and characteristic mineral component. 
These varieties of gneiss are seldom found in large masses free 
from intermixture with other sorts, but the different facies or varieties 
occur in tabular masses which are interlayered both on a large and 
on a small scale. 
These gneisses, with few exceptions, correspond accurately in 
their mineralogical and chemical composition with common types of 
coarse-grained igneous rocks like the granites and diorites. The 
light-colored granitoid rocks included under the names of Losee 
gneiss and Byram gneiss are present, in the largest amounts. There 
can be little doubt that they solidified in part out of invading silicate 
solutions or molten magmas. Evidence of crushing in the minerals 
of these gneisses is almost entirely wanting, and appearances strongly 
favor the belief that the gneissic foliation is original in these intrusive 
rocks of the pre-Cambrian complex. 
The dark Pochuck gneisses have the composition of igneous 
diorites or gabbros, but whether they have been derived from igneous 
or sedimentary originals, or, as is believed, in part from both, 
their present characteristics have in most places been acquired by 
metamorphism, involving secondary crystallization. Foliation is 
everywhere present in these dark rocks, and parallel to this structure 
they are injected in all proportions by sheets of light-colored material 
