PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 531 



Conglomerate. Diabase.] 



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



:< The sections are green and composed of fragments of andesyte, andesitic 

 rntf/ites, destroyed and blackened hornblendes, and a mineral which is pleochroic 

 varying from yellow to yellowish brown and yellowish green. Most of its sections 

 extinguish parallel to a nicol diagonal, but not all, and it appears to answer in a 

 great measure to the rhombic pyroxene of the andesytes partially altered to horn- 

 blende. The extinction of the clear, pale-yellow augite is oblique, and this is altered 

 only to a greenish cMoritp. The feldspars are altered to the usual chloritic and 

 micaceous scales. Some epidote and quartz fragments were seen. The andesitic 

 base is changed to a fibrous or granular material but is not altered as much 

 as in the preceding described andesytes, This rock apparently was once an 

 andesitic tuff or volcanic ash, as it closely resembles the modern andesitic tuffs of 

 California. 



"Plate XII, figure 1, shows in some measure the characters of this rock, and 

 exhibits, a little to the left of the centre, one of the partially destroyed hornblendes 

 so common in andesitic rocks." 



Two sections examined. 



Age. Keewatin. u. s. G. 



No. 754. CONGLOMERATE. 



"One mile north of the central narrows of Ogishke Muncie lake;' 1 probably near the south line of sec. 14, 

 T. 65-6 W. 



Kef. Annual Report, x, pages 94, 95. 



Meg. The hand specimen in general resembles the other conglomerate samples 

 already described (Nos. 738 and 744), but it is darker colored and is more indurate 

 or metamorphosed. A number of pebbles, most of them well rounded, are present. 

 These pebbles are of red jaspilyte, quartz, "greenstone," granite and hard, siliceous, 

 gray and black, flinty rocks. 



Mic. Under the microscope the rock does not differ markedly from Nos. 738 and 

 744 except that many of the fragments, especially the quartz grains, are well rounded. 

 Considerable calcite, acting as a cement, is present. 



Two sections examined. 



Aye. Ogishke conglomerate at the base of the Upper Keewatiu. u. s. a. 



No. 755. DIABASE. 



South of Ogishke Muncie lake, probably near the centre of sec. 35, T. 65-C W. Rock of one of the subor- 

 dinate hills, near the summit. 



Ref. Annual Report, x, pages 96, 97; Bulletin ii, pages 118, 119. 



A rather fine-grained, dark, greenish-gray diabase, evidently with its 

 augite altered to hornblende. 



*Bulletin ii, p. 128. 



