360 K. BELL — PRE-PALEOZOIC DECAY OF CRYSTALLINE ROCKS. 



places on this island, where the rock surface has been sheltered from 

 glacial action, whether level, sloping or perpendicular, it is eroded into 

 pits, hollows, hummocks and knobs and has a very rough appearance. 

 These surfaces bear a strong resemblance, on a large scale, to the pitted 

 exterior of an aerolite. This is well shown in figure 1, a drawing from a 

 photograph, which represents a granite surface on Benjamin island. 



The hollows- sometimes take the form of long but not deep caves, under- 

 mining the perpendicular faces of steps in the rock which rise at inter- 

 vals of ten to twenty feet from one horizontal joint to the next above. 

 At other times the}^ are sack-shaped cavities, extending from a few inches 

 to several feet into the rock. As on George island, already referred to. 



Figure 1. — Eroded Granite Surface near the north End of Benjamin Island. 



this form is very common on steeply sloping surfaces, and in these cases 

 the upper side of each cavern is generally covered with an arch which 

 thins away toward the front (figure 2). 



In half a dozen cases observed on Benjamin island, the pits at various 

 heights from the level of the lake up to 100 feet or more were filled in 

 situ with horizontal layers of the fossiliferous Black river limestone like 

 that which overlies the granite at lower levels. The limestone entered 

 into all the little irregularities of the pits and adhered firmly to their 

 walls, thus helping to retain its hold in these cavities during the time 

 the present surface has been exposed to the weather. The little patches 

 constitute veritable, if minute, inliers of the Black river formation. The 

 limestone probably originally filled all the inequalities in the granite 



