1894. ] NEW YORK ACADEMY OF SCIENCES. go 
ably spinel, as it is similar in color and refraction to the larger 
spinel grains, and, when coarse enough, which is not usual, 
appears isotropic under crossed nicols. 
The hypersthene zone is often wanting; the spinel is variable 
in amount and often disappears. But the actinolite seems to be 
very constant. The outer zone often surrounds the large hypers- 
thene grains,and sometimes, also, the monoclinic pyroxene. Its 
actinolite cannot be distinguished in appearance from the same 
mineral occurring as an undoubted secondary product, replacing 
the dark silicates and also in part the feldspar of the rock. 
Further, as spinel is abundant in the dioritic rock to which 
Z S| gh ~ <LE 
Zs Z ESSN 
Fic. 4. Reaction Rim IN OLIVINE GABBRO. 
Spec. 1552. Enlarged 43 diam. Oolivine; P—plagioclase; 
A=actinolite; H-=hypersthene. 
much of the gabbro has been altered, and is comparatively rare 
in the freshest gabbro, and associated always with the actino- 
lite, it is probably mostly, if not all, secondary, and that of the 
reaction rims has presumably the same origin. The actinolite 
rim follows the outline of the olivine, often running up into 
cracks traversing the mineral, where the hypersthene zone is 
not seen. Small plates and needles of magnetite, very like the 
secondary magnetite between the alteration cracks in the olivine, 
occur commonly between the actinolite fibres, their shape being 
