Graywacke. Argillyte.] 



PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



883 



No. 485. GRAYWACKE. (Fine.) 



Knife falls, in the St. Louis river. This rock is jointed, hard, and dike-like in appearance. 

 Ref. Annual Report, x, page 23. 



Meg. A fine-grained rock very similar to No. 480. 



Mic. The section shows a fine-grained gray wacke of the same general characters 

 as others from this vicinity. It is similar to, but finer grained, than No. 480, which 

 see. 



One section. 



Age. Keewatin(?) 



u. s. G. 



No. 486. GRAYWACKE. 



From the quartzyte spit below Knife falls, St. Louis river. 

 Ref. Annual Report, x, page 23. 



Meg. A greenish-gray, medium-grained graywacke, similar to No. 479. 

 Mic. The section shows the same general characters as already described, 

 quartz and feldspar grains in a dirty greenish groundmass. 

 One section. 



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



No. 487. ARGILLYTE. ( Slaty, j 



Knife falls, from slaty alternations in the spit below the falls. 

 Ref. Annual Report, x, page 23. 



Meg. The specimen is a fine-grained argillyte, showing the curious banding 

 described in the remark under No. 481, which might be mistaken for a sedimentary 

 structure. There is simply a zigzag grain or fibosity which by its oblique intersection 

 on the face of the cleavage planes produces a series of straight minute elevations, in 

 the form of ridges, alternating with depressions. When the light strikes these 

 favorably they cannot be seen at all, but when the ridges cast shadows over the little 

 troughs the banding is conspicuous. There is no variation in the size of the grain, 

 nor in its composition. The structure must be attributed to successive slippings, 

 accompanied by crushing pressure, the resulting motion being minute and permeating 

 the whole rock, so as to displace the original structure, and affecting the rock in a 

 sort of undulatory fracture along parallel planes. 



Mic. It is only when the section is very thin that the ultimate composition of 

 this rock can be seen. It is then found to embrace only the usual minerals of the 

 graywackes, but in a finer condition. 



One section. 



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



No. 488. GRAYWACKE. (Coarser.) 



Near the lower end of a small island north of Homestead island, St. Louis river, above Knife falls. 

 Ref. Annual Report, x, pages 23, 24, 29. 



