ORTHOCLASE-FREE GABBRO. 43 
catches in every direction the flashes of the brassy diallage faces, which are 
often as much as two inches across. 
In the thin slice the augitic ingredient presents a wine-colored or violet 
section, varying considerably in depth of tint. It often shows the charac- 
teristic prismatic cleavage, but more commonly is traversed by irregular 
cracks, or is affected in very varying degree by the diallage cleavage, parallel 
to the clinodiagonal. It is often quite fresh, but often also has undergone 
alteration, generally to some sort of soft, greenish, chloritic substance, which 
is feebly dichroic, or non-dichroic, in the section. This change is at times 
complete, no augite cores remaining. The alteration-product is often 
stained brown from the peroxidation of the iron of the augite, but in other 
cases has a pale-green color. These light-green areas sometimes contain 
particles of magnetite in such a way as to suggest that they, too, are sec- 
ondary products from the augitic alteration. The change of augite to ura- 
lite, so characteristic of the orthoclase-gabbros of the Keweenaw Series, is 
only very rarely seen in the rocks of this class, and then with an inconsider- 
able development. 
In only two or three sections out of the forty studied of this class of 
rocks was any apatite found, and then only in rare and minute crystals, 
although search was made for it in every one of the sections. A few scales 
of biotite were occasionally found. 
The existence in these rocks of serpentine, chlorite, viridite and the 
brown and red oxides of iron as secondary or alteration-products has been 
mentioned. These ingredients rarely exert any considerable influence on the 
outward appearance, the rocks of this kind being on the whole remarkably 
fresh. Prehnite as an alteration-product of the plagioclase is exceptional, 
as stated above. 
The rocks of this class present in the field very massive exposures, often 
with very marked columnar structure. They occur in very heavy beds, and 
are never furnished with an upper amygdaloidal or vesicular portion. 
As typical and easily accessible localities for these rocks may be men- 
tioned the cliffs of the North Shore, between Sucker and Knife River Bays; 
again for some five miles southward and four miles northeast of the hamlet 
of Beaver Bay; and again for long distances between the mouth of Brulé 
