712 THE GEOLOGY OF MINNESOTA. 



[Dioryte. Flint. Conglomerate. 



indicates that most of the others, foregoing, in which the origin is doubtful, and the 

 rock is much more changed, were probably not originally massive, but are rather of 

 the nature of rock No. 1395, *. e., clastic. N. H. w. 



No. 1387. DIORYTE. (Greenstone.) 



From the north slope of the same hill as No. 1386. 

 Ref. Annual Report, xvi, pages 99, 124. 



Meg. Finer grained. 



Mic. The ophitic structure is less evident, but in all other respects this rock 

 does not essentially differ from No. 1386. Between the twin lamella? of the feldspars 

 is frequently a thin layer of the hornblendic material. One section. 



Age. Archean (Keewatin). N. H. w. 



No. 1391. FLINT. (Gritty.) 



From one of the more northerly subordinate hills north of East Twin mountain. 

 Ref. Annual Report, xvi, pages 100, 124. 



Meg. Appearing quartzose, the weathered surface showing whitened pebbly 

 forms of old feldspars. 



Mic. The most conspicuous feature is the angular quartzes and micro-gran ulit- 

 ized old feldspars, which lie in a fine matrix made up of greenstone debris, f eld- 

 spathic, chloritic and leucoxenic. In high power this darker debris can be resolved 

 sufficiently to disclose also the characters of actinolite. The old feldspars are so 

 completely lost that the mass appears flinty. They appear distinct on removing the 

 upper nicol. One section. 



Age. Evidently a part of the Ogishke conglomerate. N. H. w. 



No. 1392. CONGLOMERATE. (Greenstone.) 



From the same subordinate hill range, between Ogishke Muncie lake and East Twin mountain. 

 Ref. Annual Report, xvi, pages 100, 124. 



Meg. Imperfectly porphyritic, fine grained, siliceous. 



Mic. Of the two sections with this number, one is similar to, but more appa- 

 rently fragmental than, some of the Kekequabic porphyry, except that the ferro- 

 magnesian element is wholly altered to irregularly spreading and often branching 

 masses of hornblendic material, and is also scattered as a chloritic powder throughout 

 the slide. There is some calcite and much quartz in the fine groundmass, which 

 otherwise, along with some feldspar, has a granulitic structure, causing the rock to 

 resemble the porphyry referred to. The greater decay of the augitic element, and 

 the rather scant or wholly absent secondary growths about the large feldspar crystals, 

 ally it, however, to the conglomerates about the west end of Ogishke Muncie lake, 

 which also contain debrisjfrom some porphyry. 



