Gabbroid Rocks of Minnesota. — Wincluil. 369 
Pectolite occurs in small quantity in only one type, the 
olivine gabbro, where it is clearly derived from the direct de- 
composition of the feldspar. 
AntJiophyllite is another secondary product ; it is derived 
from the enstatite or bronzite of the cordierite noryte. All 
stages of the transformation occur, from the compact pyroxene 
to the fibrous or lamellar amphibole. The two m.inerals often 
have a common orientation, and simultaneous extinction. 
Anthophyllite contains rather feeble, greenish pleochroic halos 
about inclusions of zircon. 
Actinolite and tronolite are rare alteration products found 
in the silicoferrolyte and orthoclase gabbro. 
Hornblende is abundant in the orthoclase gabbro ; in the 
ordinary gabbros and diabases it is rare ; in the other types it 
is wanting, though it probably existed at one time in the 
quartz gabbro. It is secondary in all these rocks. 
Allanite, as well as apatite, sometimes causes pleochroic 
halos in hornblende. 
Chrysolite occurs in the gabbros and diabases ; it is the 
most abundant mineral of the troctolyte ; in the silicoferrolyte 
it is replaced by fayalite ; in the quartz gabbro, it is only rep- 
resented by rare masses of bowlingite, nearly amorphous ; in 
the plagioclasyte, orthoclase gabbro, and cordierite noryte it is 
entirely wanting. It seems to be highly ferriferous, the ratio 
Fe : Mg varying from about i : i .4 in the olivine gabbro to 
I : .8 in the olivine diabase, and the mineral is thus even too 
rich in iron for typical hyalosiderite. It is said to be replaced 
occasionaHy by fayalite, even in the gabbros. 
Fayalite occurs only in the silicoferrolyte, where it is abun- 
dant. The optic properties alone, notably the negative sign 
of the bisectrix and the smaller angle of the oi)tic axes dis- 
tinguish it from chrysolite. The cleavages are more conspicu-- 
ous than in unaltered chrysolite. 
Epidote occurs very rarely in the cordierite nor}te. where 
it seems to be of primary origin. It occurs still more rarely 
in the orthoclase gabbro, and in the (|uartz gabl.iro. wlierc it 
is undoubtedly secondary. 
Allanite occurs in the Pigeon point diabase, and very rarely 
in some samples of the normal gabbro ; it forms small crystals, 
sometimes rounded or irregular. In color and pleochroism. 
