392 



THE VERMILION IRON-BEARING DISTRICT. 



the ridges and intervening low ground present an appearance very similar 



~ to that of the teeth of a saw, and from this circumstance 



•I they are sometimes called sawtooth mountains. Starting 



^ at the north at a lake one ascends a steep ridge rising 



I 200 or 300 feet above the lake in many instances, then 



I descends the gentle dip slope to the south, which leads 



I down to a second depression occupied hj a lake, then 



S ascends again a steep northward-facing hill with gentle 



M southerly slope, and so on. Dolerite sills occur interca- 



I lated in these Rove slates and usually cap the hills. 



I Their influence on the topography is referred to later on 



I (p. 400). The topographic character of that portion of 



%. Minnesota underlain by the Rove formation can be seen 



i in PI. XIII, A, and fig. 21 in the text. 



•v.! 



STRUCTURE. 



^1 



rA 



The structure of the slates in this area is very 

 simple. Wherever they have been examined they are 

 found to have a very uniform dip of from 5° to 25° to 

 the south-southeast. They evidently form a part of the 

 great monoclinal series of slates which are known as far 

 west as the Mississippi River in the Mesabi district, and 

 which continue directly east, cross the east end of the 

 Vermilion district, appear on the north shore of Lake 

 Superior north of Grand Portage, and continue thence 

 eastward around Thunder Bay and northeastward along 

 the shore of Lake Superior for some distance. As indi- 

 cated by the variation in dip — from 6° to 25° — the 

 monocline of slates is occasionally interrupted by minor 

 rolls, which, though of little importance, can be noted by 

 close examination of almost any of the great cliffs that 

 give good exposures. 



PETROGRAPHIC CHARACTERS. 



The slates form the bulk of the Rove formation, but 

 with them are associated graywackes, some quite slaty, 

 otliers very massive, and also some fairly pure quartzites. 



