184 



MINERALS AND GEOLOGY 



project from the face of cliffs or hill-sides, or stand up above the 

 general surface of the ground, and thus resemble in many cases the 

 stone fences or walls known in certain localities as " dykes/' The 

 annexed figure exhibits a diagram-view the surrounding foliage, 

 <fec., being omitted of a pro- 

 jecting dyke of this kind, as 

 seen on Slate River, a small 

 rocky stream which enters the 

 Kaministiquia about twelve 

 miles above Fort William on 

 Thunder Bay, Lake Superior. 

 The high cliffs of aluminous 

 slate or shale on each side of 

 the ravine through which the 

 river flows, have been wasted 

 by atmospheric action to a 

 much greater extent than the Fi - 102< 



dyke ; and the latter thus stands out from the face of the cliff on 

 each side of the ravine, and presents the appearance of an old Gothic 

 wall. On one side, it comes down close to the bank of the stream, 

 as seen in the figure ; and the arch, there shewn, must have been 

 hollowed out, when the water flowed with fuller volume and at a 

 somewhat higher level, during the gradual excavation of the valley. 

 Most of the projecting points, reefs, and rocky islets on the shores of 

 our northern lakes, consist of denuded portions of trappean dykes. 

 Occasionally, however, trap and greenstones decompose more readily 

 than the surrounding or encasing rock. Trench-like depressions in 

 the ground, or clefts and open fissures on the face of the rock, are 

 thus produced. Examples may be seen on some of the islands and 

 parts of the coast of Lake Superior, near JSTeepigon Bay, and else- 

 where. 



Traps and greenstones occur also, in many districts, in the form of 

 flat tabular masses, resting upon hill-tops. These are merely por- 

 tions of ancient dykes, exposed and isolated by denudation. Finally, 

 mountain-masses composed of trappean and greenstone rocks are of 

 frequent occurrence, but these also may be regarded in most cases as 

 the more salient portions of enormous dykes, several being often 

 seen to lie in the same general direction, as though along an ex- 



