PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 905 



Diabase. Muscovadyto.] 



from a molten magma in the places in which they now lie in proximity to the slate 

 with which the intrusive is now in contact. In other words the porphyry was not a 

 molten mass when intruded into the slate, but simply a plastic mass, moved by 

 pressure, suffering internal fracture and subsequent recrystallization. It might be 

 called a secondary intrusion, but hardly an igneous rock. What may have been the 

 age and origin of this rock at first, it is impossible to state from any evidence in the 

 rock itself, but from general considerations and from alliances which it shows with 

 other Keewatin " porphyries," it is reasonable to assume that it dates from some 

 earlier portion of the Archean. It may have been at first a porphyrel like that of 

 Zeta lake. N. H. w. 



No. 798G. DIABASE. ( Coarse grained.} 



From the centre of a dike at least 150 yards wide and running nearly north and south, near the south 

 line of sec. 20, T. 65-6 W., between Knife and Epsilon lakes. 

 Ref. Annual Report, xxi, page 62. 



Meg. A coarse-grained, somewhat uralitized, greenish diabase. The feldspars 

 are also somewhat greenish in color. 



Mic. The section shows a coarse-grained diabase, the essential minerals of 

 which are plagioclase, augite and iron ore. The plagioclase (species not carefully 

 determined, but apparently near labradorite) is much clouded by kaolinic alteration. 

 The augite is reddish in color and very slightly pleochroic. It occurs in large 

 dphitic grains and there is more fresh augite than would be supposed when one 

 considers the altered nature of the feldspar. The augite has, however, altered in 

 many places to a more or less confused, fibrous, greenish and yellowish aggregate, 

 in which can be recognized some green hornblende, some chlorite and a little biotite. 

 The iron ore is accompanied by opaque gray alteration products (leucoxene) and is 

 thus shown to be ilmenite. A little secondary quartz is present. One section. 



Age. Cabotian(?) 



Remark. This rock (also No. 799G) is somewhat similar to and comes from the 

 same general locality (Knife lake) as Nos. 1742 to 1751. u. s. G. . 



No. 799G. DIABASE. (Olimnitic. ) 



Near the centre of the east side of N. W. % sec. 7, T. 65-6 W., Amoeba lake, at the portage to Knife lake. 

 Ref. Annual Report, xxi, page 62. 



Meg. A coarse-grained diabase, with gray to greenish feldspar. 

 Mic. The section is quite similar to that of No. 798G. No. 791)0, however, has 

 a little olivine. One section. 



Age. Cabotian(?) u. s. G. 



No. 847G. MUSCOVADYTE. (Fi-iifjnii'nt. in gabbro.J 



S. W. J4 N. W. % sec. (i, T. 04 r>. Island in Gabemichigama lake. 



Ref. Annual Report, xxi, page page 65 ; Annual Report, xxiv, page 127 ; Annual Report, xv, pages 171, 

 1 T2 ; Final Report, vol. iv, figure 43. 



