74 THE CRYSTAL FALLS lEOX-BEARISG DISTKICT, 



bends to the southeast, and continues with this general trend for about 16 

 miles into T. 42 N., R. 31 W., where my field study of it ended. At the 

 north the belt runs into the eastern half of the district described by Smyth, 

 and swings south, which coui'se is followed for some 15 miles. The 

 entire belt thus forms an oval siu-roundiug the sedimentaries, except in the 

 southeastern part of the district. Another area of Hemlock volcanics is 

 found in T. 43, Rs. 32 and 33, just north of Crystal Falls. This area is 

 about one-half a mile wide just north of the city of Crystal Falls, but rap- 

 idly widens as it is followed to the west until at the western limits of the 

 area it is about 3^ miles wide. A third small isolated area is found in sees. 

 17, IS, 19, and 20, T. 42 N , R. 32 W , and sec. 24, T. 42 N., R. 33 W., 

 about 4 miles south of Crystal Falls. 



The topography of the Hemlock formation is exceedingly rough where- 

 ever erosion has succeeded in cutting tlnough the drift mantle. This occurs 

 only adjacent to some of the streams. The rough topography at these 

 places is due to differential erosion working upon rocks approximately on 

 edge, and of varying hardness. The valleys visually indicate the location 

 of beds of tuff and the higher grounds are almost universally occupied 

 by dense rocks forming the lava flows, or of the coarse-grained massive 

 intrusive i-ocks. In a few places, however, the thoroughly consolidated and 

 indurated tuffs form high hills. In traversing the Hemlock formation one 

 makes an abrupt ascent, followed by a sharp descent into a narrow swamp, 

 then another ascent, and so on. Exposures appear for the most part in 

 small areas along the edges of the swamps and scattered over the faces of 

 the hills. These are fairly numerous, but so small and disconnected as to 

 prevent the tracing out of the individual flows, although this might be pos- 

 sible if the traverses were made at very short intervals and the area mapped 

 in great detail. 



THICKNESS. 



As has been seen, the belt of eruptives varies in width from one- 

 half of a mile to nearly five miles. The dip of the rocks is about 75° W. 

 The enormous thickness of 25,500 feet which these data would give is 

 probably illusory. 



In the case of the assumption of the thickness of a series of lava flows 

 and tuffs, it is important that the initial dip, which these deposits must have, 

 be considered. This dip varies greatly, depending on the slope- of the 



