384 THE GEOLOGY OP MINNESOTA. 



[Graywacke. Diabase. 



Meg. A coarser graywacke similar to Nos. 469, 477, 478. 



Mic. The section shows the usual grains of quartz and feldspar in a rather 

 sparse groundmass of chlorite, calcite, sericite and finer quartz and feldspar. A few 

 small grains of a yellowish mineral with a high index of refraction and strong double 

 refraction are present; this seems to be epidote. There is also a rock fragment 

 similar to the finest-grained of these graywackes. This rock shows more rounded 

 and sub-angular grains than, and not so many sharply angular ones as, most of the 

 rocks from this vicinity. Moreover, the outlines of the rounded grains are distinct 

 and they do not interlock with the adjacent grains as in No. 473. 



Two sections. 



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



No. 489. GRAYWACKE. 



From a large boulder near the same locality as No. 488. 

 Sef. Annual Keport, x, page 24. 



Meg. Finer grained than the last, and intermediate between it and the more 

 slaty portions. Similar to No. 486. 



Mic. One of the usual graywackes of medium grain. 

 One section. 



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



No. 490. GRAYWACKE. 



From the rapids in the middle channel, north of Island No. 5, near Knife falls. 

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



Meg. The hand specimen and the section are closely similar to No. 489. 

 One section. 



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



No. 491. GRAYWACKE. 



From the same place as No. 490. 

 Ref. Annual Report, x, page 24. 



Meg. An arenaceous, somewhat slaty, hard gray rock. 



Mic. The section shows the same composition. 



One section. 



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



No. 492. DIABASE (with olivine). 



From large boulders at the rapids at the head of Island No. 4, apparently from a large dike in place near, 

 but which cannot be seen. 



Ref. Annual Report, x, pages 24, 29; Bulletin ii, page 108. 



Meg. Gray, rather coarse-grained and fresh, heavy rock. 



Mic. The feldspar is ophitically embraced by the augite. Olivine is not 

 abundant. Magnetite and a small quantity of alteration products, these constitute 



