THE 
AMERICAN GEOLOGIST, 
Vol. XXVI. OCTOBER, 1900. No. 4 
MINERALOGICAL AND PETROGRAPHIC STUDY 
OF THE GABBROID ROCKS OF MINNESOTA, 
AND MORE PARTICULARLY, OF . 
THE PLAGIOCLASYTES.* 
(Plate XX.) 
[Continued. See other plates in the September Number. 1 
By Alexander N. Winchell, Butte, Mont. 
Chapter III. Olivine Diabase. 
The olivine diabase is entirely similar in method of occur- 
rence to the normal diabase; indeed the intrusive gabbro, like 
the deepseated gabbro much oftener contains olivine than 
not. The same columnar structure is every where found in 
the olivine diabase, which occurs in the same general geo- 
graphical areas and in the same geological relations as the 
normal diabase. 
The typical sample for this chapter came from the extreme 
eastern part of the area of gabbros in Minnesota; it is derived 
in fact, from the extremity of Pigeon point. It probably 
formed part of one of the numerous "Logan sills," of the north 
coast of lake Superior, though diabase masses of other origin 
are said to exist in the region of Pigeon point. The contact 
area which is also found here presents intense metamorphic 
phenomena, but the sample studied does not belong to the 
contact zone. 
In color the olivine diabase is a mottled gray always green- 
