210 Wisconsin Academy of Sciences , Arts and Letters. 
the olivine, and it is probable that in some cases at least the hornblende 
is an intermediate stage, the alumina necessary for the alteration being; 
obtained from feldspar. Cores of olivine are found in some crystals 
that are more than half altered. In the second of the two sections no 
olivine is present, but patches of more or less opaque green or brown 
substance indicate the position of the original mineral. 
In both sections a little biotite is present in fresh blades which give 
the marked mottled appearance just before extinction between crossed 
Nichols. Apatite is abundant in crystals of moderate size, sometimes, 
though rarely, broken across. Fluid inclusions in the apatite are rare. 
Magnetite is quite abundant in large crystals which frequently show 
skeleton forms, and there is a little pyrite. 
The rock is therefore a coarse-grained and feldspathic olivine diabase.. 
Examination of a larger number of sections would quite likely add some 
characters to those that have been mentioned. The presence of a tal- 
cose mineral in the decomposed rock observed at the locality is not ex¬ 
plained by examination of the sections. It doubtless arises mainly from 
a profound alteration of the pyroxene constituent, which in the sections 
is almost unaltered. 
