376 C. W. HALL — KEEWATIN OF EASTERN AND CENTRAL MINNESOTA 



Plate 31. — Slate Quarry and Graywacke Exposure 



Figure 1. — Slate quarry at Thomson, Minnesota. 



The slate dips strongly southward. The diagonal lines across the 

 face of the quarry represent the bedding planes. Lenticular 

 nodules of sideritic material lying along these lines liave assumed 

 a vertical position conforming with the slaty cleavage. This 

 cleavage can be seen on the left of the picture above the hammer. 

 Photograph by C. P. Berkey. 



Figure 2. — Shattered surface of graywacke and graywacke slate, Thomson, Min- 

 nesota. 



The dip is southward. In these rocks also the carbonate nodules 

 have been squeezed into a vertical position. These are well shown 

 on the knob to the right. Near the middle of the picture is a finely 

 carved glacial groove whose wearing is in the direction of the bed- 

 ding of the rocks. Photograph by C. P. Perkey. 



Plate 32. — Grayu)acke Slaie and Graywacke 



Figure 1. — Exposed surface of graywacke and graywacke slate near Thomson, 

 Minnesota. 

 The rock is thoroughly jointed, and occasionally some displacement 

 is seen. This fracturing was probably produced coincident with 

 the production of the slaty cleavage seen in neighboring clay slates. 

 Surface produced by glaciation. Photograph by C. P. Berkey. 



Figure 2. — Graywacke with contorted quartz vein, Carlton. Minnesota. 



This view is from the railroad cut south of Carlton, Minnesota, and 

 shows a greatly contorted quartz vein one to three inches wide. 

 The gray wac ke is quite compact and free from the sideritic nodules 

 characteristic of the exposures at Thomson, as shown on the pre- 

 ceding plate. Photograph by C. P. Berkey. 



