386 THE GEOLOGY OP MINNESOTA. 



[Graywacke. Argillyte . 



No. 497. GRAYWACKE. 



"From near the middle of the central channel a short distance above the head of the uppermost Chain 

 island." 



Kef. Annual Report, x, pages 26, 29. 



Meg. A rather light gray graywacke of medium grain. It is quite similar to 

 Nos. 496 and 493, but finer grained than the latter. 



Mic. The section shows, as usual, quartz and feldspar grains in a greenish 

 groundmass. 



One section. 



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



No. 498. ARGILLYTE. 



The rock of the country 150 yards above the fall where No. 495 was obtained. 

 Kef. Annual Report, x, page 26. 



Meg. Gray, fine-grained, firm, cleaved argillyte. 



Mir. This rock has no representing section. It is of a lighter gray color on 

 the edges of the laminae than on the sides. It is plainly too siliceous to make a 

 roofing slate. 



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



No. 499. GRAYWACKE. 



"Rock immediately overlying No. 498." 

 Kef. Annual Report, x, page 26. 



Meg. A rather fine-grained graywacke. A few small pieces of a black argillyte 

 are seen in the hand sample. 



Mic. The section shows the usual graywacke described more fully under Nos. 

 473 and 488. 



One section. 



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



No. 500. GRAYWACKE. (Slaty.) 



From near the southwest corner sec. 18, T. 49-1G. From just below the falls which here occur in the St. 

 Louis river. 



Kef. Annual Report, x, page 26. 



Meg. Fine grained, gray. 



Mic. Besides the usual minerals this section shows a few grains that are much 

 oxidized and stained with limonite, which were probably sidcritc. There is also one 

 large oblong area which is filled with finely granular quartz grains, and appears to 

 be of the well-known jaspilyte, or at least like that rock. 



One section. 



Ayr. Archean (Keewatin). 



Remark . This grain of jaspilyte shows that as a rock it must have pre-existed. 

 It could not, therefore, have been as late as those granites which are supposed to 



