A TYPE OF IGNEOUS DIFFERENTIATION 633 
hand specimens can hardly be trimmed from it. A striking local 
variation contains long needles of dark minerals in a red matrix. 
In thin sections it is micropegmatitic, varying to granitoid in some 
large masses. Muarolitic cavities are numerous in some places. 
Variability is as characteristic of the minerals as of the textures. 
The chief red mineral is a feldspar stained with considerable hema- 
tite and badly kaolinized. Probably most red rock contains two 
feldspars; zoning is especially common in the phases grading into 
the gabbro. Quartz, though abundant, is rarely visible except with 
the microscope as an intergrowth. Hornblende is the chief ferro- 
magnesian mineral, but it is fibrous and mixed with secondary 
minerals as if itself secondary. Biotite is rare and in most cases 
secondary. 
A sample of the red rock was selected from the type locality 
for analysis (number 25 of the table), and while it cannot fully 
represent so variable a rock, it shows some features in common with 
earlier analysis also quoted in the table. Nearly all the norms 
include corundum; many also include hematite. For a rock con- 
sisting largely of graphic intergrowth of quartz and feldspar— 
supposed eutectic proportions—the quartz is high and alkalies are 
low. The lime and potash, both being low, make the rock resemble 
bostonite in composition, but it is more quartzose than that type. 
Winchell’ has tabulated the terms used in the common qualitative 
system for the red rocks. Possibly granophyr is appropriate for 
most of the rock. 
Gradation and relations.—In the sill in the eastern part of the 
city there is a remarkable example of perfect gradation from dia- 
base to red rock. The diabase is of ordinary type, with a finer 
contact phase at the base. It is exposed almost continuously for 
a width of a mile, equivalent to a thickness of several hundred feet. 
The diabase grades up into a red-rock zone of smaller thickness 
and less regularity, though a belt may be followed several blocks. 
It is noteworthy that while the sill must be nearly 1,500 feet thick, 
the conspicuous gradation zone is less than 50 feet, from black 
diabase to intensely red granophyr. 
tA. N. Winchell, ‘‘Review of Nomenclature of Keweenawan Igneous Rocks,”’ 
Jour. Geol., XVI (1908), 765-74. 
