PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 909 



Gabbro. Quartz- porphyry.] 



ularly amongst the feldspars, it does not surround them ophitically. The slide con- 

 tains very little olivine. There is a small amount that surrounds in a narrow rim 

 some of the magnetites, but it is so small that it is only accessory, the rock being 

 composed essentially of diallage, feldspar and magnetite, one-half being diallage. 

 One section. 



Age. Cabotian. N. H. w. 



No. 989aG. GABBRO. (Almost anorttosyte. ) 



Same place as No. 989G, and interbanded with it. 



Ref. Annual Report, xxii, page 84 ; Final Report, vol. iv, page 489. 



| 



Meg. Coarsely crystalline, and consisting largely of gray feldspar. 



Mic. The minerals show a succession in the date of formation. The magnetite 

 and coarse feldspars were the first to crystallize. Next came the diallage which is 

 not abundant, or rather small in amount, occupying the open angular spaces left 

 between the feldspars. If it had been in sufficient amount the diallage would have 

 embraced the feldspars ophitically. The slide contains no olivine. The diallage is 

 largely altered to hornblende. One section. 



Age. Cabotian. 



Remark. With some qualification this rock might be styled a diabase, since its 

 structure is essentially ophitic. N. H. w. , 



ROCKS COLLECTED BY A. WINCHELL. 



No. 24W. QUARTZ-PORPHYRY. 



S. W. y N. W. M sec. 3, T. 62-14, eastern extremity of Little Mud lake. 

 Ref. Annual Report, xvi, page 27. (Compare No. 376.) 



Meg. Light gray, massive or coarsely schistose. 



Mic. In a very fine, interlocking and rather uniform groundmass of. feldspar, 

 quartz, Muscovite, chlorite and calcite are large feldspars of idiomorphic form, which 

 are themselves so crowded with the same minerals in fine granules that they can 

 hardly be distinguished from the rest of the section. It is only by rotation of the 

 stage between the nicols that it can be seen, at the points of greatest light, that 

 feldspar forms are concealed in the section. There are also a few irregular patches, 

 of considerable size, of chlorite which has doubtless resulted from an alteration of 

 hornblende. Apatite in a few small crystals, and sphene smaller and fewer, can be 



