362 The American Geologist. December, 1900 
pyroxene and olivine lead to the melanokratic type, peridotytc^ 
which is very rare, and no specimen has been available for this 
study. 
When the labradorite increases in abundance, the rock be- 
comes generally of a lighter color, the specific gravity dimin- 
ishes, and the grain increases in coarseness. At the same time 
the olivine seems to disappear entirely, while the pyroxene 
becomes so rare that one can scarcely find it without the mi- 
croscope. The relative amount of magnetite and of apatite 
may rest about the same, or decrease somewhat. Fresh frac- 
tures present many large cleavage faces of feldspar which are 
bright and transparent, and very often show the fine striations 
characteristic of the albite twinning. It is thus that the gab- 
bro changes to plagioclasyte, the leucokratic type of the gab- 
bro group. This rock, which is very abundant in large areas 
in America, and notably in northern ^Minnesota, constitutes 
generally only a mineralogical accident in the European gab- 
bro regions. 
Third type' of variatioii. The origin of the qiimtz gabbro 
is somewhat obscure, but in all probability it may be classed 
among the products of contact metamorphism. 
In external characters the quartz gabbro is markedly less 
firm, and more easily crumbled. It has a lighter color and a 
distinctly lower specific gravity. The texture remains unal- 
tered, but the grain may increase in size, or porphyritic varie- 
ties may appear. Besides the minerals of the normal gabbro, 
quartz and zircon occur as primary constituents. The miner- 
alogical and chemical composition is thus changed both quan- 
titatively and qualitatively. This is the acid extremiC of the 
series studied. The pyroxene, as noted, has entirely disap- 
peared. The feldspar being anandesine, is more acid than in 
the gabbro. It is often much altered. A nearly amorphous 
bowlingite is the present representative of the original oliv- 
ine. Magnetite probably occurred in small amount as an 
original constituent, but that which is now present seems to 
have, had a secondary origin. Apatite occurs in unusually 
large crystals, but sparsely distributed. Calcite and rutile oc- 
cur as secondary products. 
It is possible that the orthoclasc gabbro is the result of the 
