Gabbroid Rocks of Minnesota. — Winchell. 209 
opaque mineral, probably magnetite; it is quite possible that 
these are of secondary origin ; liquid inclusions have also been 
observed, but not commonly. 
The altcratio7is of the olivine diabase have been described 
by Bayley* ; the results of this alteration are the same as those 
produced by the alteration of the normal gabbro with the ex- 
ception of pectolite and the addition of titanite. These 
minerals are the following: 
Calcite. Hematite. 
Muscovite (?) Pyrite. 
Penninite. Bowlingite. 
Clinochlore. Antigorite. 
Hornblende. Titanite. 
Magnetite. Biotite. 
Secondaky Mixerals. Calcite presents no unusual features; 
it forms at the expense of labradorite by endogenesis. It is 
easily recognized by its extreme birefringence and its uniaxial 
negative character. 
Muscovite is exceedingly difficult to distinguish under the 
microscope from certain other minerals, especially paragonite. 
margarite, pyrophyllite and talc. The measure of the optic 
angle will usually distinguish it from all except paragonite; 
but this measure is often impossible, especially when the 
mineral occurs, as in the diabases, in the form of microscopic 
scales much too small to give a satisfactory interference figure. 
Nevertheless it is probable that the mineral is a muscovite 
(perhaps damourite) since it is a matter of frequent observa- 
tion that the result of the process of sericitization, is nearly al- 
ways muscovite even in the basic rocks, where the process is 
comparatively rare, probably from lack of potassium. 
The mineral under discussion is eminently lamellar in habit, 
and shows a perfect cleavage parallel to the faces of the 
lamellae. It is transparent and colorless. The refringence is 
always superior to that of the labradorite. The birefringence 
is very strong since the thin scales show colors as high as 
green of the second order. The elongation of the lamellae 
is always positive, with extinction sensibly parallel. 
*W. S. Bayley: Eruptive and sedimentary rocks en Pigeon point. 
Minn., and their contact phenomena. U. S. Geol. Surv. Bull. No. 109, 
1893. 
