518 THE GEOLOGY OP MINNESOTA. 



[Andesyte. Gabbro. 



No. 734. ANDESYTE (icith hornhii'mii ). 



Near the same place as No. 733. 



Ref. Annual Report, x, page 89; Bulletin ii, page 127, plate XI, ttgure 2. 



Meg. A fine-grained, greenish-gray rock, containing a few porphyritic crystals 

 of white feldspar and of hornblende. 



Mir. The section is quite similar to the preceding (No. 733), but that section 

 is not porphyritic. The porphyritic feldspars are considerably altered, but are 

 clearly plagioclase. The porphyritic Itonibh-iules are sometimes fibrous and some-- 

 times compact, but do not have sharp crystal boundaries. Some of these hornblendes 

 are brownish in color, and there are narrow green rims around some of them. Zonal 

 structure, aside from the rims, is also seen, and the ends of even the compact grains 

 are more or less fibrous. An area of caldte was seen. 



Two sections examined. 



A(jc. Archean, Lower Keewatin or older. u. s. G. 



No. 735. GABBRO. (Allcmi.) 



Kails at the outlet of Prog Rock lake, near the centre of sec. 17, T. (M f> W. 

 /iV/'. Annual Report, x, page 89; Bulletin ii, page 120. 



Meg. A medium-grained, gray rock, looking like a dioryte. The minerals are 

 white feldspar, a greenish- black mineral and a little pyrite. 



Mic. M. E. Wadsworth's description of this slide is as follow*:* 



" The section is gray and of a granitic structure. It is composed of gray, gran- 

 ular, altered feldspars, with crystals of partially altered aiigife. 



" The augite is partly replaced by uralite, chlorite, etc. Some quartz, epidote&nd 

 colorless mica plates were seen." 



One section examined. 



Age. Archean, Lower Keewatin or older. r. s. G. 



No. 736. ANDESYTE (ivith homlj/cnile). 



From 'an island in Frog Rock lake, about half a mile southwest from its outlet; N. W. % S- W. sec. 17, 

 T.65-5W. 



Ref. Annual Report, x, page 89; Bulletin ii, pages 125, 126, plate XI, h'gure 1. 



Meg. A fine-grained, greenish-gray rock, with small porphyritic hornblende 

 crystals. 



Mic. M. E. Wadsworth's description of this section is as follows:! 



" The section has a greenish groundmass holding yellowish-green and greenish 

 pseudomorphs after pyroxene and perhaps hornblende. The pseudomorphs are 

 composed chiefly of hornblende, biotite and epidote. One twinned crystal, apparently 



'linltrtin ii, p. 120. 

 ^Bulletin ii, p. 128. 



