PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 501 



Gabbro. Cumberlamlyte.] 



.1%. A medium-grained, gray, granitoid rock, composed of plagioclase, biotite, 

 titaniferous magnetite and other black material. A small amount of a bright yellow 

 mineral (probably chalcopyrite) is present. 



Mic. M. E. Wadsworth's description of this rock is as follows:* 



"In the thin section the pyro.mi<> appears to have been principally altered to a 

 fine matted mass of adinolite fibres and to biotite; but the plagioclase has suffered 

 less change than would naturally be supposed from the amount of alteration in 

 the pyroxene. Considerable biotite of a secondary origin is to be seen fringing the 

 magnetite grains. The usual needles and inclusions common in the feldspar of 

 gabbros occur here in a manner that indicates that they are the result of the com- 

 mencement of the plagioclastic alteration rather than original forms." 



The rock is clearly a gabbro. The lioni/ifei/ile, chlorite and part of the magnetite 

 and biotite are evidently secondary minerals from the original augite.noneof which 

 remains. The feldspar is near l<il>ni<l<-it<\ as is shown by its extinction angles in 

 sections normal to 010 and in cleavage fragments. 



One section examined. 



Age. Cabotian. u. s. G. 



No. 702. GABBRO. 



Same locality as No. 701. " No. 702 is embraced in pockets and lumps in No. 701. It is the same rock but 

 with much coarser crystals and a larger percentage of diallage (see No. 088 and No. ID)." 

 Ref. Annual Report, x, pages 78, 82, 84; Bulletin ii, pages 70, 71. 



Meg. A coarse-grained granitoid rock, composed of whitish feldspar, magnetite 

 and augite, the last mineral being in large masses and especially abundant. 



Mic. This rock affords an illustration of very coarse dialhujc associated with 

 plagioclase said nuiyitefitc, all in large grains, the first especially being in large masses. 

 The diallage appears to have formed not much later than the feldspars, and has more 

 or less curving outlines, yet embraces occasionally small portions of the plagioclase. 

 llroini ( " basaltic" ) hornlhmle, much resembling biotite, is closely associated with 

 the magnetite about its borders. There is also a small amount of biotite, common 

 hornblende and elilm-ile. 



One section. 



Age. Cabotian. N. H. w. 



No. "Ii;!. CUMBERLANDYTE. 



Iron ore, at the cast end of Pewabic island, in Mayhew lake. 



Kef. Annual Report, x, page 82; Bulletin ii, pages 89, 90, plate IV, figure 1. 



Meg. Titanic iron ore, mingled largely with pyroxene. 



Mic. The fettt*/>itr has extinction on 11, at 22 and on 010 at 25. It is entirely 

 surrounded sometimes by the magnetite, and is separated from it by a border of brown 



*Bu2iettn ii, p. 72. 



