370 The American Geologist. December, i9co 
as well as in birefringence, the mineral is quite variable. It 
causes intense pleochroic halos in all the colored minerals in 
which it has been observed. 
Penninite occurs in small amount in every type studied, 
except the orthoclase gabbro and the cordierite noryte. It is 
most abundant in the most highly altered type, the quartz 
gabbro, where the positive and negative varieties both occur. 
It is derived from the alteration of the original ferromagnesian 
element of the rock, which was probably a pyroxene. 
Clinochlore occurs in every type except the cordierite nor- 
yte, quartz gabbro, and troctolyte ; thus the only type contain- 
ing no chlorite is the cordierite noryte. The pleochroism of 
clinochlore is very similar to that of positive penninite. Cli- 
nochlore is usually positive. It is derived from the alteration 
of pyroxene, or some other ferromagnesian mineral. It seems 
to occur sometimes as an alteration product of feldspar. 
Antigorite is f-are in Minnesota gabbros, but occurs spar- 
ingly in the normal gabbros, and in the diabases. It is 
formed from the decomposition of olivine, but is usually de- 
posited away from the olivine grains in the fractures (and cavi- 
ties, if any exist) in the surrounding minerals, 
Boivliyigite is comparatively abundant, and is beautifully 
developed in the olivine diabase from Pigeon point, where it is 
perfectly crystallized, and often oriented on the olivine. It is 
formed also from fayalite in the silicoferrolyte. 
Apatite occurs in every type, except the troctolyte and sili- 
coferrolyte, but it is abundant only in the orthoclase gabbro 
and quartz gabbro. 
Titaiiite occurs as a decomposition product in the olivine 
diabase and in the orthoclase gabbro. 
Zircon has not been found in the normal gabbros, nor in 
any of their unmetamorphosed forms ; it occurs only in the 
cndomorphosed types, such as the cordierite noryte, and the 
quartz gabbro. In these rocks it occurs in the usual, small, 
square prisms which determine intense halos in penninite and 
biotite and feebler ones in anthophyllite. 
Spinel occurs only as a thin coating over the magnetite, 
when it is surrounded by cordierite. It is j^robably the variety 
pleonaste. 
Magnetite occurs as a primary mineral in every type 
