PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 305 



Gabbro.] 



Meg. A very fine-grained, gray or greenish gray, fissile slate. Contains some 

 micaceous mineral along the cleavage planes. 

 No section. 



Age. Animikie. u. s. G. 



No. 300. GABBKO (with hornblende and quartz). 



From a hill S. W. J sec. 30, T. 65-3 E. South of Moose lake (of the international boundary). Rises 485 

 feet above Moose lake. One of the common coarse eruptives of the region. 



Ref. Annual Report, ix, page 76; Annual Report, x, page 84; Bulletin ii, page 80, plate VII, figure 1. 



Mac. Coarse, gray, apparently altered basic irruptive, containing hornblende. 



Mic. The feldspar is changed by the entrance of micropegmatitic quartz; in 

 some crystals the quartz occupies one-third of the total area. -Micaceous and 

 ferruginous products of decay cloud the feldspar, but in some grains its triclinic 

 character is still manifest. Extinction on 010 indicates a labradorite near andesine. 

 Extinction on a section perpendicular to n s is 22, indicating labradorite adjoining 

 andesine. (Fouque.) 



In the decay of the augite, it seems to have changed first to "diallage," and then 

 to hornblende. In the diagram presented by Wadsworth, this transition is well 

 shown. (Bulletin ii, plate VII, figure 1, and page 80.) The cleavages of the augite 

 are all parallel, indicating a section in the prism zone. These lines are marked and 

 continuous, though not rigidly straight. The fibrous intercalation, however, which 

 marks the progress of decay and the orientation of the " diallage," as shown by him, 

 forms an angle of about 72 with the cleavage of the augite, which is very near the 

 angle /?, i. e., the angle between the base and the orthopinacoid. It appears, hence, 

 that the fibrous disintegration, which follows certain coarse cracks, is parallel to the 

 base of the augite crystal. Its origin, in this case, therefore differs from that which 

 is assigned to the diallagic schillerization by Judd, not only in its cause, but also in 

 its direction in the crystal (Quarterly Journal, Geology Society, xlii [1886], page 82). 

 This change is here probably one that results from weathering. The whole section 

 indicates a weathered condition of the rock from which it is obtained. The diallagic 

 parting which is found in the older -augites of the gabbros is seen in numerous 

 cases, and is parallel to the orthopinacoid. It is a strong cleavage-like parting, not 

 fibrous nor lacking in transparency. Such crystals are found, not where weathering 

 has affected them, nor dynamic pressure, but in the deep-seated and most protected 

 portions of the gabbro masses. This parting coexists with the prismatic cleavage. 

 It seems probable that two different forms of alteration in augite may have been 

 confounded, one due to the cooling stage of the rock, when gases and hot solutions 

 permeated it, and the other due to ordinary weathering. They seem to differ not only 

 in the direction in which the lamellation grows in the original crystal, but in the 

 cause which produced them, and also in the degree of integrity which is preserved 

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