EVIDENCES OF THE DECAY. 359 



'' ovens," which may also be adopted for convenience as their geologic 

 designation. The oven shown in this figure measures 5 feet deep, 8 feet 

 wide and 2f feet high. Two men can crouch inside of it. The door or 

 entrance is 18 inches high and 2f feet wide, which are also about the 

 proportions of the door of a baking oven of the above dimensions. 



The j)its, which occur mostly upon the nearly horizontal surface of the 

 granite, differ from glacial kettle-holes in being shallower, seldom quite 

 circular and in having rough walls. The sack-shaped ovens are most nu- 

 merous upon sloping surfaces. In nearly every case the fundus of these 

 hollows is pointed up-hill, arched over by a roof having a thin edge o ' 

 rather harder granite than the general mass, and is in the form of a seg- 

 ment of a circle. The granite around these pits and caverns is all bare 

 and exposed to the weather, but it shows no decay or disintegration 

 below the immediate surface. Figure 2 of plate 15, the reproduction of 

 a photograph taken at Killarney village, represents this condition. In 

 some places near by it is partially smoothed and striated, as if the glacier 

 had touched it very lightly and planed off little spots here and there, the 

 fine striae running south 80° west, but at half a mile to the eastward the 

 granite is completely moutonne and the grooves run south 40° west. 



Rocks of the Huronian Belt. 



The portion of lake Huron l)etween the main north shore and the 

 Manitoulin chain of islands is called the North channel. The quartzites, 

 schists, etcetera, of the great Huronian belt, standing nearly on edge, 

 strike very regularly about east and west for many miles along the north 

 shore and through numerous islands of this channel. A few miles west 

 of La Cloche an intrusive mass of red granite, having a nearly circular 

 form and a diameter of about four miles, interrupts the run of the 

 quartzites, etcetera, which, however, continue their regular east-and-west 

 course on either side of it. On Benjamin island the hills in the central 

 part of the granite intrusion rise to a height of 160 feet above lake Huron. 

 Resting on the southern flanks of this island are small areas of Black 

 river limestone near the level of the lake, while larger flat-lying por- 

 tions of the same formation, constituting Amedros, Clapperton and Hook 

 islands, approach the granite closely to the east, south and west. 



Ancient Erosion. 



Typical Erosion Surface. — Benjamin island, which is about two miles 

 long, consists of hummocks of granite of various heights up to 160 feet, 

 and these present bluffs toward all points of the compass. In many 



