EROSION. 361 



surface on which it was deposited, but along with the great mass of the 

 formation was removed by denuding agencies, mostly prior to the glacial 

 epoch. 



Figure 1 of plate 16 represents some arenaceous limestone remains in 

 situ, in one of the pits, on a steeph^ sloping granite surface at the south 

 end of Benjamin island. The eroded surface of the rock on this island 

 also occasionally exhibits curious low, circular ridges three or four feet 

 in diameter and three or four inches in height, weathered out of the 

 granite (figure 3). Each circle is surrounded by a slight depression, 

 while the space within has the form of a very shallow basin with a small 

 mound rising in the center. The granite of the mound and the annular 



Gaverrv a/ony joi/it p/cui^. 



liffieffoney u/ysi/io; 



Gtxtnil» -' ^ " J 



Figure 2. — Section of eroded Surface of Granite loith Lower Silurian Limestone resting upon it. 



ridge are rather finer grained than the surrounding mass. They are ap- 

 parently of concretionary origin. 



Conditions affecting Erosion. — Under what conditions did the ancient 

 erosion above described take place? The facts above presented, as well 

 as others about to be mentioned, would appear to indicate that it oc- 

 curred at the bottom of the deep sea, for it is known that rock-surfaces 

 do sometimes become pitted or dissolved under deep water. If this 

 erosion had taken place on land we should have evidence of deeper 

 decay in the substance of the rock, while the subsidence of the land to 

 permit the deposition of the fossiliferous limestone upon it would be 

 indicated by detrital shallow-water deposits, such as sandstones and 



