PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 533 



Dioryte. Limestone.] 



grains, etc., occur with the hornblende as secondary products. Leucoxene occurs with 

 the iron ore, which is arranged in dash-like bars. These bars form rectangles and 

 rhombs with one another one perfect rhomb measuring for its inside angles, approxi- 

 mately, 70 30' and 109 30'. Several other imperfect rhombs yielded on measure- 

 ment about the same angle." One section examined. 







Aye. Keewatin. u. s. G. 



No. 759. DIOHYTE. (Gabbroitl) 



"Occurs in patches in No. 758, and is like No. 756, but contains so much magnetite as to appreciably increase 

 its weight." 



Ref. Annual Report, x, page 97; Bulletin ii, pages 77, 78. 



Meg. A coarse-grained, granitoid rock, dark greenish-gray in color and composed 

 of hornblende, feldspar and some magnetite. 



Mic. The section , which appears to have been originally a gabbro or coarse 

 diabase, is composed essentially of secondary green hornblende, chlorite, considerably 

 altered plagioclqse, magnetite and secondary quartz. No augite remains. 



One section examined. 



Age. Keewatin. u. s. G. 



No. 760. LIMESTONE. 



Specimens NOB. 700 -7(!5 were obtained on a trip taken to Twin peaks from Ogishke Muncie lake. The 

 trip was probably made from the west end of this lake in S. W. J4 sec. 27, T. 65-6 W., through W. J^ sec. 34, T. 

 65-6 W., to the summit of East Twin peaks, which is probably in N. E. ^ sec. 4, T. 64-6 W. The following notes 

 wore taken on the specimens collected on this trip:* 



" In passing from Ogishke Muncie lake directly to Twin peaks, the following observations were made by 

 Mr. Mallmann. After a space of conglomerate, perhaps one-half a mile, there is a gorge in which a creek runs 

 S. 30 W., the rock over which it passes being: 



" No. 760. A light siliceous marble, or calcareous quartzyte, undistinguishable from the marble, No. 746. 

 On either side of the gorge is a wall of rock. On the west is conglomerate, and on the east is 



" No. 761, which is a pyritiferous, dark, aphanitic rock, one of the conditions of the slate of the slaty 

 conglomerate. 



" No. 762. Is from a trap dike cutting the conglomerate, about one-fourth mile from the lake. 



" South from the gorge is more conglomerate and quartzyte, then a ridge, or a succession of benches rapidly 

 ascending. The first bench, twenty feet high, has a regular bedding, and dip toward the south, but no basaltic 

 structure. It consists of 



" No. 763. Which is apparently closely allied to No. 755. 



" No. 764. In a higher ridge is this rock, which is the same as No. 763. 



"No. 765. The top of the Twin peak (East Twin) is a fine-grained variety of the same rock. This peak, 

 and the general average of the mountain range, may be 500 feet higher than Ogishke Muncie lake. The Twin 

 peaks are conspicuous because they stand isolated, further north than the rest of the high land, not because 

 they are higher." 



Ref. Annual Report, x, page 97. 



Meg. A gray siliceous limestone very similar to No. 746. 



Mir. The section is in general similar to that of No. 746, but contains more 

 cryptocrystalline quartz. Chlorite and some opaque gray material are present. There 

 are also angular fragments of quartz and feldspar, and in some parts of the section 

 there is a resemblance to a crushed and altered quartz-porphyry. 



One section examined. 



"Tenth Annual Report, p. 07. 



