PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 385 



Graywacke. Diabase.] 



the rock, which is similar to many other dike rocks already noted, in the same 

 region. 



One section. 



Age. Cabotian dike in the Keewatin(?) N. H. w. 



No. 493. GRAYWACKE. 



At the rapids at the head of Island No. 4. From boulders, but evidently from the formation in place at 

 this point. 



Ref. Annual Report, x, pages 24, 26. 



Meg. A coarse, gray, harsh, graywacke. 



Mic. The description of No. 473 or of No. 488, will apply to this rock, but it is 

 rather more coarse than they. 

 One section. 



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



No. 494. GRAYWACKE. 



About half a mile below Knife falls at the river bank. Prom one of the firmer beds of the formation. 

 Ref. Annual Report, x, pages 24, 26. 



Meg. Schistose, gray, ordinary graywacke. 



Mic. The section shows the usual ingredients, also some large areas of calcite 

 or of siderite. 



One section. 



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



No. 495. DIABASE. 



From a dike 200 feet wide. Fall in the St. Louis river near southwest corner of sec. 18, T. 49-16 W. 

 Ref. Annual Report, x, pages 25, 26. 



Meg. A medium-grained, gray, heavy diabase. 



Mic. The section is quite thick. It shows an ordinary diabase with considerable 

 of a green alteration product. 

 One section. 

 Age. Cabotian dike in Keewatin(?) rocks. u. s. o. 



No. 496. GRAYWACKE. 



At the head of the uppermost of the Chain islands above Knife falls. Same as No. 493. 

 Ref. Annual Report, x, page 26. 



Meg. Gray, medium grained. 



Mic. The quartz snad feldspar grains show the same sub-angular shapes. They 

 lie in a gray matrix of chloritic substance in which are finer grains of the same 

 minerals, with some calcite. 



One section. 



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



2fi 



