296 The American Geologist. November, 1900 
seen. When fresh the rock is remarkably tough and tenaci- 
ous. On exposure to the weather it becomes yellowish brown 
to dark greenish brown, through the oxidation of the iron. 
At the same time it loses in density to a remarkable degree, 
and often becomes so soft that it can be crumbled easily with 
a hammer. 
Under the microscope the texture is seen to be finely 
granular and poikilitic. (See plate V, figure i.) The latter 
texture is especially noticeable in the relation between the 
biotite and cordierite, but with the nicols it is discovered that 
both the former, and the bronzite are enclosed by large quartz 
grains which are poikilitic in their relation to the other 
minerals, but xenomorphic in their relations to one another. 
Feldspar is so rare that it has little effect upon the texture of 
the rock as a whole; the four common minerals are cordierite, 
bronzite, biotite, and quartz. Of these the cordierite is the 
oldest, and countless grains of it are included within each of 
the other three; bronzite is next in age and while containing 
grains of cordierite it is often itself surrounded by biotite 
or quartz. Biotite always formed before the quartz, and is 
often enclosed by it. 
The accessory minerals of the rock include enstatite, mag- 
netite, pyrite, apatite, zircon, staurolite,* and, more rarely, 
epidote and spinel. The only products of posterior decom- 
position observed are anthophyllite, muscovite, and an un- 
determined mineral. 
Study of the Minerals. The dominant plagioclase 
of the noryte is slightly more acid than the labradorite of 
the normal gabbro. It is in much smaller grains, and much 
less abundant in the noryte; crystal outline is wanting entire- 
ly. The albite and pericline types of twining occur. The 
acute bisectrix is n^_ ; the optic angle is large. The extinction 
in S;ig is 19° — 22°; T;/p it is 63° — 64°; the maximum equal 
extinction on both sides of the albite twining line exceeds 30° ; 
the extinction perpendicular to one of the optic axes is some- 
what variable, being about 38^-7-41°; the extinction in cleavage 
pieces parallel to /(ooi) is 5° to 6°, and parallel X.og^{o\<S) it is 
20° to 22°. The composition of the labradorite is therefore 
near Abi Ani. 
*Contains graphite inclusions. 
