430 E. S. MOORE 



evidences of differentiation which have already been mentioned 

 by Dr. A. P. Coleman and the writer and to add additional notes 

 to the descriptions of this phenomenon. 



PETROGRAPHY OF THE DIABASES 



The Keweenawan rocks around Lake Superior have been 

 described petrographically in detail, by Irving, Bay ley, Van Hise, 

 and many others. In the vicinity of Lake Nipigon the rocks are 

 in many respects similar to those around Lake Superior and they 

 have been described with less detail by Coleman, Wilson, and other 

 geologists. The greater portion of the shores and the islands of 

 this lake consist of basic rock, either diabase or gabbro. Thin 

 sections almost invariably show the ophitic texture more or less 

 well developed, and, although in many places the diabase grades 

 toward gabbro, the greater portion of the rock is diabase. In 

 the sills, diabase always seems to be found, and the same state- 

 ment may be made of the smaller bodies of the rock, while some of 

 the larger batholithic masses, which have suffered some differ- 

 entiation, more strongly resemble gabbro. 



Structurally the rocks form bosses, large and small, batholiths, 

 or very large irregular masses, dikes, and sills. The dikes are 

 often large, as some were seen in the Onaman Iron Range area 

 150 ft. in width, and these seem to represent offshoots from the 

 main diabase mass in the vicinity of the lake. The sills, known 

 as the "Logan sills," form beds from two to several hundred feet 

 in thickness. These masses lie between beds of sandstone, shale, 

 or dolomitic limestone, or between these sediments and the under- 

 lying Archean rocks, and in all cases studied they present evidence 

 of their intrusive character. Columnar structure is a character- 

 istic of nearly all of the larger masses, especially of the larger sills. 



In macroscopical characters these basic rocks generally present 

 a monotonous appearance. They vary in grain from coarse to 

 medium fine and in color from brownish to nearly black. Some 

 of them weather rapidly to granular incoherent masses, and, in the 

 early stages of this weathering, they exhibit in many places cleav- 

 age surfaces with a bronze tint. In many cases the ophitic texture 

 is readily recognized in the hand specimen, but in the masses 



