PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 519 



Conglomerate.] 



originally of rhombic and monoclinic pyroxene, has been altered to yellowish horn- 

 blende with the dichroism varying from pale yellow to yellowish green. This shows 

 the cleavage of hornblende while the prismatic planes are those of pyroxene. See 

 plate XI, figure 1. 



"The pseudomorphs show, as a rule, regular forms, but the chlorite, hornblende, 

 and epidote also occur in irregular masses scattered through the groundmass and are 

 formed from the molecular aggregation of their material from the groundmass and 

 not from any direct pseudomorphic process. 



; ' The epidote is abundant, and the groundmass of this section is composed of 

 tabular altered feldspar (largely plagioclase) in a firmer groundmass of felty material, 

 colorless mica and chlorite scales, epidote, microliths and grayish altered magnetite. 

 The feldspars are mainly changed to chlorite and colorless mica scales, while the 

 mica, in quite large plates, is associated with the epidote. 



" The original condition of this rock was apparently that of an andesyte of the 

 augite-andesyte type, and in its altered condition it is now best classified as a por- 

 phyryte, or, if preferable, a horneblende porphyryte or even a dioryte. 



" The analysis given below, made by professors Dodge and Sidener, of this rock, 

 shows that chemically.it is the same as an altered basalt, and hence it may be that 

 my reading of the microscopic physical characters is incorrect, if one was to be 

 governed by the chemical composition alone in deciding." 



SiO 2 49.65 



A1 2 O 3 16.36 



Fe 2 O 3 4.39 



PeO 7.19 



CaO 9.18 



MgO 8.00 



Na 2 O - . 2.49 



K 2 1.17 



H 2 O - ... 2.39 



Total - 100.82 



One section examined. 



Age. Archean, Lower Keewatiu or older. u. s. G. 



No. 737. CONGLOMERATE. 



"A taleose or chloritic rook, having a coarse perpendicular schistose structure, from the rapids between 

 Frog Rock and Town Line lakes, northwest from Frog Rock lake. This is eonglorneritic, some of the pebbles 

 being six inches across, and like the gray amphibolyte (No. 731). There is in it also considerable pyrite. The 

 pebbles are rounded, and are intimately connected with the matrix. There may be thirty feet of this here 

 visible, including that below the water and above. There seems to be a south dip of about 10 to 15. The 

 aspect of the country changes at once on entering on this conglomerate. There are no bald rock hills to be 

 seen around the Town Line lake, but the trees grow all over the rock and down to the water." N. W. 14 S. W. 

 14 sec. 18, T. 65-5 W. 



Ref. Annual Report, x, pages 89-91, 95; Bulletin ii, pages 121, 122. 



Meg. The few pebbles seen in the hand sample are of " greenstone;" the matrix 

 varies in color from green to pinkish gray, is rather soft, fine grained, and somewhat 

 schistose. 



