364 The American Geologist. December, 1900 
The first noticeable external change is in the coarseness of 
grain, which becomes much reduced and is very fine in most 
of the norytes, and cordierite norytes. In the former the text- 
ure usually remains granitoid, but in the latter it becomes 
poikilitic. The color of the rock becomes more uniform on 
account of the fine texture, and is often slightly darker gray, 
or green. The specific gravity remains practically constant, 
but the rock becomes exceedingly tough, and difficult to 
break. Nevertheless, it alters much more readily than the true 
gabbro, and quickly changes to a crumbling mass of very fine 
grains. This is largely due to the fact that cordierite is a 
very unstable mineral, and its alteration products are found 
much oftener than the fresh mineral. 
Microscopically, the first change from the normal gabbro 
type is the appearance of hypersthene, or enstatite, which grad- 
ually replaces the augite. At the same time the texture be- 
comes much finer, and chrysolite is often replaced by fayalite, 
or may disappear entirely. Biotite is nearly always one of the 
new minerals. The feldspar may become slightly more acid. 
When this chang-e is complete the noryte type is reached. 
Quartz norytes occasionally occur. , 
The next step is the appearance of cordierite, which is al- 
ways in very fine rounded grains or crystals. In this type the 
olivine is absent ; the pyroxene is enstatite or bronzite in small 
spheroidal masses. Biotite is abundant, enclosing the cordier- 
ite and enstatite grains. The feldspar is an acid labradorite, 
or even andesine, but may become rather uncommon ; quartz 
commonly occurs, and may be abundant. The accessory min- 
erals are numerous and include: magnetite, pyrite, apatite, 
zircon, epidote, staurolite, spinel, and probably graphite. An- 
thophyllite is a notable alteration product. 
In resume this comparative study makes it evident that 
the rock types studied can be grouped in the following way : 
Typical normal rock — Olivine gabbro. 
I. Variations due to conditions of solidification. 
Intrusive. — Coarse olivine diabase. "1 
\ porphyritic varieties. 
' Dykes, etc. — Fine diabase to basalt. J 
II. Variations due to differentiation or to the original 
heterogeneity of the magma. 
