698 THE JOURNAL OF GEOLOGY. 



It has already been intimated that the normal rock of the 

 great gabbro is so uniform in its general character that, after 

 studying carefully one of its hand specimens, others might 

 easily be identified among a collection of specimens of the basic 

 rocks of the Lake Superior region, without much danger of error. 

 Its description, therefore, is quite a simple matter. In its macro- 

 scopic aspect the normal rock is a medium to coarse-grained, gray, 

 granular aggregate of a very lustrous plagioclase and a black 

 augite. The plagioclase is usually more abundant than the darker 

 mineral ; its dimensions are larger, and its contours more frequently 

 approximate to those of crystals. It is of a light gray color and 

 has a glassy lustre on fresh fractures, while on weathered surfaces 

 it is white and opaque. Twinning striations are visible on nearly 

 every grain. The augite on the contrary is jet black. Its cleav- 

 age faces are rather small, and its contours never approach those 

 of crystals ; they are occasionally triangular or wedge-shaped 

 when they have any definite form, but are usually very irregular 

 in outline. In some of the coarse-grained varieties of the rock 

 there is a rudely lamellar arrangement of both the augite and the 

 feldspar, so that the mass possesses a platy structure. With this 

 exception the gabbro has the typical granitic texture, and is thus 

 easily distinguished from all the other so-called flow gabbros of 

 northeastern Minnesota and the region bordering on Lake Supe- 

 rior in which is more or less perfectly developed the diabasic 

 texture. 



The principal varietal differences noted in the rock are due 

 solely to the proportions of feldspar, augite and olivine present 

 in it. When the pyroxene is in moderate quantity the appearance 

 of the specimen is as indicated above. Sometimes the feldspar 

 is largely in excess, and pyroxene has almost entirely disappeared. 

 Now the rock has a lighter gray color, and the bright shining 

 black particles are lacking. Again olivine is the principal com- 

 ponent when the tint of the rock becomes dark green. The 

 structure in all cases, however, remains the same. The varieties 

 are merely local phases of the predominant rock for on all sides 

 they grade into one another by insensible transitions. The 



