THE BA SIC MA SSI VE RO CKS, ETC. 819 



per cent, to 16.03 P er cent, of Ti0 3 . He gives no account of 

 their associations, but refers to them simply as gabbro-magnetites. 

 In the same group he includes the magnetite occurring as sand 

 on Black Beach, near Beaver Bay. The rock of Beaver Bay is, 

 however, not a portion of the great gabbro mass that we are now 

 discussing, but it is probably a portion of a sheet or " sill " intru- 

 sive in the Keweenawan strata, and occurring at a much higher 

 horizon than that at which the rock which is the subject of the 

 present series of papers, occurs. 



Olivine-Pyroxene Aggregates. — When magnetite is scarce in 

 the basal beds of the gabbro, the rocks are usually very fine- 

 grained, sugary aggregates of yellowish green olivine, and a dark 

 brilliant substance that upon examination is found to be a 

 pyroxene. Occasionally a large plate of pyroxene or of 

 hornblende is imbedded in the fine aggregate, but this so 

 rarely happens that it does not affect the general appearance 

 of the rocks. On the other hand it is not uncommon to find in 

 what is apparently a fine-grained mass many large areas of irreg- 

 ular shapes that reflect light uniformly. These evenly reflecting 

 surfaces are the cleavage faces of large grains of pyroxene or of 

 amphibole that are so completely saturated with inclusions as to 

 appear as heterogeneous in structure as the aggregate in which 

 they lie. The entire fracture surface of specimen M. 453H, 

 for instance, is made up of interlocking areas with evenly reflecting 

 surfaces, each covering about a sixteenth of a square inch, and yet 

 so full of little inclusions are they that the rock must be described 

 as possessing a finely granular texture. In very rare cases the 

 thinner bands are very coarsely granular, when they consist 

 almost exclusively of pyroxene with now and then a little mag- 

 netite. In all cases the structure is obscurely schistose, and 

 large pieces tend to break more easily along their bedding planes 

 than transversely thereto. 



In the most olivinitic phases of these rocks olivine comprises 

 nearly their entire mass. The mineral here occurs in irregular 

 interlocking grains of a pale yellowish green color, crossed by 

 cracks and fissures in which a little limonite or other brown iron 



