38 THE VERMILION IRON-BEARING DISTRICT. 



Grant describes the gabbro area as an elevated plateau, upon whose surface, 

 however, there are many minor irregularities, but few of which rise 100 

 feet above the surrounding country. Lakes are common, but very shallow. 

 "The general plain-like character of the gabbro-covered area can be 

 ascribed to weathering, erosion, and glaciation acting on a surface composed 

 of a single rock mass (the gabbro) uniform in constitution, grain, and 

 resistance to disintegrating agents."" 



Gimflint Lake area. — Near the east end of the district, as shown on 

 the map (PI. II), there is an area having approximately the shape of an 

 isosceles triangle, with the apex of the triangle pointing west. The north 

 side is bounded by the Giants range, the south side by the gabbro 

 plateau, and the base is the eastern limit of the region shown on the 

 general map. Within this area a very interesting kind of topography is 

 developed. The following descrijDtion is based upon a personal visit to 

 the greater portion of the area described, and upon the published reports of 

 Dr. U. S. Grant." 



This area is underlain by a series of Upper Huronian slates which 

 have a low dip to the south-southeast. The continuity of the slate series 

 is interrupted by numerous sills of coarse dolerite of varying thickness, 

 which were intruded approximately parallel to the beds. Pre-Glacial 

 erosion developed here a system of east-west ridges and valleys. Each 

 ridge is capped by a layer of dolerite and below, protected by this hard 

 upper layer, lie the slates. The ridges slope gently to the south. The 

 slope follows approximately the upper side of the dolerite sill and corre- 

 sponds closely to the dip of the sediments. To the north the ridges break 

 off abruptly, giving a steep or precipitous escarpment with a talus below. 

 The narrow valleys between the ridges are usually occupied by lakes, and 

 each lake is higher than the one in the valley next north of it. A cross 

 section would show on the north a talus surmounted by a cliff which forms 

 the brow of the ridge, the latter sloping gently to the south to the valley, 

 which contains a lake. Then the talus, cliff, and slope are repeated. Such 

 a north-south section would have much the jagged appearance of the edge 

 of a saw. From this character these hills are frequently spoken of as the 

 "sawtooth hills." 



«Grant, op. cit, p. 435. , 6 Op. cit., pp. 317, 482. 



