198 The American Geologist. October, i9(X) 
ish; besides the minerals seen in normal diabase green olivine 
occurs, usually abundantly. 
On decomposition the rock becomes darker as the olivine 
alters to bowlingite, etc., and the pyroxene to hornblende; 
bitotite often appears also. 
Texture — Microscopically the ophitic texture is very 
prominent (see plate IX, figure 2), and the order of crystalliza- 
tion is: 
Apatite 
Accessory 
Magnetite, AUanite 
t Labradorite 
Essential •] Olivine 
( Pyroxene 
Pyrite and ilmenite also occur; they crystallized in general 
at the same time as the magnetite. Finally traces of vermicu- 
lar quartz have been found. 
The relative time of consolidation of the allanite is not well 
known ; it is rare and has only been found enclosed in pyroxene 
and secondary minerals; it may therefore be of formation 
somewhat later than indicated above. The other minerals 
formed in the order indicated with the usual exceptions due 
to the fact that one period of crystallization is scarcely ever 
completed before the next begins. The labradorite seems 
to have crystallized almost wholly before the pyroxene, but 
its relation to the olivine is not so well defined. The latter 
has sometimes a rough outline nearly its own, but oftener 
it molds itself around sharply defined crystals of feldspar and 
sometimes it may be found in triangular areas limited by the 
plagioclase crystals. Thus while in part cotemporary with 
the latter the olivine is in general slightly later than the labra- 
dorite. The pyroxene in exceptional cases seems to be auto- 
morphic toward the olivine. 
Primary Minerals. The feldspar of the olivine dia- 
base is a labradorite of varying basicity. Twinning is 
very abundant, and in one thin section a single an- 
hedron shows four types, namel}- albite, Carlsbad, peri- 
cline and Baveno. This is the only case in which 
the last t}-pe of twinning has been found in this rock 
series. It occurs in a section of normal diabase (954) from 
Birch lake. (See Plate IX, Fig. i.) The acute bisectrix 
