362 K. BELL — PKE-PALEOZOIC DECAY OF CRYSTALLINE ROCKS. 



Figure 3. — Low, circular Ridge, four Feet in Diameter, 

 on weathered Surface of Granite. 



conglomerates. The occurrence of pure limestones in the pits and ovens 

 of Benjamin island, in one case in the face of a perpendicular bluff, as 



well as of the nearly horizontal 



beds of greater extent upon the 

 sound though uneven surface of 

 the granite, such as might exist 

 at great depths in the sea, but 

 not upon the land, shows that 

 the rock had become thus eroded 

 and was destitute of other cov- 

 ering when the limestone began 

 to be deposited upon it. The 

 sea bottom then rose to a more 

 moderate depth, so that an 

 abundant fauna found a suita- 

 ble habitat and the formation 

 of the limestone commenced. 

 The area now occupied by the 

 North channel of lake Huron 

 might at that time have formed 

 a quiet arm of the sea, with hills of Huronian rocks to the north, as at 

 present, and similar hills to the south, which have since sunk to lower 

 levels, but of which traces remain. 



Contacts of Archean with Paleozoic Rocks. 



Along the junction of the Archean and Paleozoic rocks occurring be- 

 tween the foot of lake Ontario and the head of Georgian bay the actual 

 contact of the Potsdam sandstone and Black river limestone with the 

 Laurentian gneiss may be seen in a great number of places. The surface 

 of the gneiss, which can be observed to pass under the nearly horizontal 

 beds of the newer rocks, is generally rough or angular, quite hard and 

 fresh looking, as if it had never been exposed to weathering on land. 

 The points and promontories of these flat-lying strata which stretch 

 northward from the general line of the basal geographic boundary of the 

 Silurian rocks in this region lie on the lowest ground or in the oro- 

 graphic depressions of the southern margin of the Archean area, where 

 they found shelter from glacial erosion or prfeglacial decay. It is doubtful 

 if either these or higher formations ever extended over a large part of 

 the Archean area to the northward, as is commonly supposed, although 

 judging from the vast amount of their ruins which have been carried 

 south they must have been much more extensive before the glacial epoch 



