642 FRANK F. GROUT 



Mottled diabase. — This is probably the most easily recognized 

 by the unskilled, from the fact that it invariably weathers to a color- 

 mottling, and usually to pits or projections on the surface (Fig. 5). 

 Colors depend upon the conditions of weathering. As a rule, the 

 flows are thick, many of them weather spheroidally, and the amygda- 

 loidal zone is not prominent. The latter point has made some 

 occurrences seem more like intrusive than extrusive rock, but no 

 such conclusion is forced upon one, here. Pumpelly's early descrip- 

 tiofi^ has been widely accepted and its outline of the development of 

 the rock is probably correct. He clearly presents the appearance of 



Fig. 6. — Conchoidally fracturing diabase. About one-half natural size 



ophitic texture and the resultant luster-mottling. The original rock 

 is reported as containing augite, plagioclase, olivine, and magnetite, 

 with evidences of glassy matrix. Apatite is rarely seen. Alteration 

 yields many other minerals. Of the fifty Minnesota rocks of this 

 type, recently examined in detail, only one section revealed olivine 

 cores of a size and freshness to yield an interference figure; but 

 nearly all were well supplied with pseudomorphs, so similar to the 

 fresh olivine as to be quite unquestionable, though the material was 

 fibrous and pleochroic. Opaque minerals develop along the borders 

 and cracks of the original grains, giving the impression of high relief 

 to the alteration products, chlorite, serpentine, and iddingsite. 



Magnetite is abundant in all sections. Hematite is prominent 



I R. Pumpelly, Geology of Wisconsin, III, ^;^. 



