PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 147 



Diabase.] 



No. 53B. DIABASE. (Modified.) 



Duluth. N. W. ) sec - 24, T. 50-14. From the longest rocky point; a part of No. 53, but here containing 

 some flesh-red crystals, making it resemble No. 5. 



Ref. Annual Report, ix, page 19; Bulletin ii, page 107. 



Mic. The section shows a rather coarsely crystalline rock quite similar to No. 

 5, except that it seems to embrace more quartz. The grains of this quartz are some- 

 times of considerable size. They are both irregular in form and also sharply angular. 

 In the latter case the quartz embraces in an ophitic manner portions of the reddened 

 feldspars and some of the greenish products of decay from other minerals. It also 

 encloses apatite and magnetite. This shows it to be of later origin than these min- 

 erals, but these minerals themselves, excepting, perhaps, the apatite, are not original, 

 as shown by numei'ous observations already noted. While taking part, therefore, in 

 the general transformation during the cooling process, quartz was the latest of the 

 secondary minerals to take its place. The presence of quartz here, and not in most 

 other cases of such change, is to be assigned, as in No. 5, to the proximity of some of 

 the clastic rocks which afforded it in connection with the general metamorphism 

 resultant from the cooling period when hot, and hence silicified waters would have 

 been abundant. 



The apatite has every appearance of being one of the earliest crystallizations. 

 It is not only in the greenish and serpentinous grains resulting from change in 

 pyroxene, and perhaps from olivine, but it is in the unchanged or comparatively pure 

 pyroxene and in the feldspars, whether the latter be reddened or entirely fresh. Its 

 spicules are large and persistent, and sometimes very long. Its sections, perpen- 

 dicular to the principal axis, are numerous and hexagonal. The fact, mentioned by 

 Wadsworth in description of No. 6, that apatite when "entirely enclosed" in sec- 

 ondary quartz is probably of secondary origin, seems not sufficient to prove it so, 

 but, on the contrary, simply shows it was of earlier date than the quartz. In the 

 section, as stated, while it is perhaps more common in the vicinity of, and embraced 

 in, the secondary minerals, it is not confined to them. Some conspicuous spicules 

 penetrate the freshest of the feldspars. Its prevalence in the decayed minerals is 

 probably due to its previous greater abundance in the pyroxenic grains. 



Biotite in small scales is rare in connection with the alteration products. 



Two sections examined. 



[Evidently Wadsworth's description assigned to No. 53B is of some other rock.} 



Age. Cabotian. 



licnim-lcx. This rock resembles No. 5. It thus connects No. 53 with the age of 

 the great gabbro, or eruptive epoch, that closed the Animikie, and in that respect 

 again points to the eruptive origin of the doubtful rock No. 52 and its equivalents, 

 as the surface expression of the great Cabotian eruptive revolution. Compare the 

 description of rocks Nos. 854G and 854aG, and the accompanying remark. N. H. w. 



