774 THE GEOLOGY OP MINNESOTA. 



[Conglomerate. Augite-syenyte. Gabbro. 



No. 1738. CONGLOMERATE. (Greenstone.) 



At the outlet of Snowbank lake. 

 Kef. Annual Report, xxi, page 157. 



Meg. Apparently like a graywacke. 



Mic. The rock is composed of feldspar fragments, often conspicuously twinned, 

 never interlocking, usually much decayed, and similar fragments of hornblende. Both 

 the feldspar and the hornblende are slightly zoned and enlarged by growths at 

 different epochs, though this feature is not conspicuous. Some quartz, apparently 

 in original clastic condition, occurs in the midst of these coarser grains. The 

 groundmass that surrounds these larger elements consists in part of fine feldspars, 

 but mostly of fine interlocking quartz and feldspar of secondary origin, the arrange- 

 ment and variation in fineness of which often show that this replacement occupies 

 the areas of some older mineral which has disappeared entirely. The rock is exactly 

 like many described about Kekequabic and Ogishke Muncie lakes, belonging to the 

 Ogishke conglomerate. One section. 



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



NO. 1739. AUGITE-SYENYTE. 



East side of sec. 31, T. 64-8, Snowbank lake. 

 Ref. Annual Report, xxi, page 157. 



Meg. Coarse, presenting the so-called bedded structure of much of that about 

 Bassimenan lake. 



Mic. The augite has a negative elongation and is hence cegyrine. It is twinned 

 on 100. The feldspar is much ingrown as a microperthite, twinned, zoned and inter- 

 locked, several different orientations frequently appearing in the same crystal, each 

 one occupying but a small area interlocking with the general crystal and with the 

 other small areas. These feldspars are oligodase, orthoclase, nitoiiJtoclase and a little 

 microcline. Biotite, apatite and sphene are also in the slide. One section. 



Age. Archean. N. H. w. 



No. 1742. GABBRO. ( Much altered. ) 



At the great Knife Lake headland, southwest corner of the headland, at the water level. 

 Ref. Annual Report, xxi, page 157. 



Meg. Rather coarse grained and gray, with evident quartz.- 



Mic. Quartz, hornblende, augite, leucoxene, biotite and an altered Jeldspar compose 

 this rock, so far as represented by the slide at hand. One (thick) section. 



Age. Cabotian(?) 



Remark. In connection with Nos. 1742 to 1751, compare Nos. 798G and 799G, 

 which are somewhat similar coarse-grained diabases from the vicinity of Knife lake. 



N. H. w. 



