PETROGRAPHIO GEOLOGY AND DESCRIPTIONS. 381 



Grraywaeke.] 



Mic. The section shows quartz and feldspar grains in the usual dirty greenish 

 groundmass. Both macroscopically and microscopically this rock is very similar to 

 No. 469. (Compare also .description of No. 473.) 



One section. 



Age. Keewatin(?) u. s. G. 



No. 478. GRAYWACKE. 



"Represents the last rock on the right [south] bank of the St. Louis above Knife falls." 

 Ref. Annual Report, x, pages 21, 23, 29. 



Meg. Rock of the same general character as No. 477. Running through it is a 

 band of decidedly coarser grain, but of the same composition. 



Mic. The section was evidently made from the coarser band. It shows the 

 usual quartz and feldspar grains in a greenish groundmass. The rock is very similar 

 to No. 477. Compare also the description of No. 469, which is a similar, but finer- 

 grained, rock. Nos. 477 and 478 differ from No. 469 in only one essential particular, 

 i. e., in lacking, or almost lacking, siderite and pyrite. 



One section. 



Age. Keewatin(?) u. s. G. 



No. 479. GRAYWACKE. 



From the brink of Knife falls, St. Louis river. 

 Ref. Annual Report, x, pages 21, 22. 



Meg. Similar to No. 477, a gray, hard, arenaceous fragmental rock. 

 Mic. The section shows no essential variation from the characters already men- 

 tioned for several graywackes. 

 One section. 



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



No. 480. GRAYWACKE. 



"Prom the brink of Knife falls." 

 Ref. Annual Report, x, page 21. 



Meg. A greenish-gray rock, much finer grained than most of the graywackes 

 already described. A few small quartz and feldspar grains can be seen in a greenish 

 background. 



Mic. The section shows a few scattered quartz and feldspar grains, much smaller 

 and fewer in number than usual in these rocks, in a greenish ground mass. The section 

 is too thick for a careful study, but it is evident that this groundmass is composed, as 

 usual, of chlorite (and probably sericite), and fine grains of quartz and feldspar. The 

 rock is thus like the other graywackes of this vicinity except that it is mostly 

 composed of the material which makes only the groundmass of the other specimens. 



One section. 



Age. Keewatin(?) u. s. G. 



