358 R. BELL — PRE-PALEOZOIC DECAY OF CRYSTALLINE ROCKS. 



yet received much explanation. Between the time of the folding of the 

 Huronian rocks and the deposition of the earliest fossiliferous beds there 

 must have been an interval much greater than is commonly supposed. 

 Throughout the vast region of Canada occupied by the Archean rocks 

 the attitude of the Huronian and Laurentian strata generally approaches 

 the vertical, and their surface has generally been cut down to nearly a 

 horizontal outline, which is only slightly raised above the sealevel. 



Lower Silurian strata, especially limestone, unaltered and full of fossils, 

 may be seen in many places, resting almost horizontally upon the up- 

 turned and denuded edges of the crystalline rocks, and yet the former 

 contain but few fragments of the older terranes, and they occur only at 

 the very contact, showing that the latter had a hard and naked surface 

 when the Paleozoic rocks were being deposited upon them. 



Evidence of pre-Paleozoic Decay. 



Where best displayed. — The evidence of pre-Paleozoic decay of the an- 

 cient surface is more discernible upon the surfaces of masses of intrusive 

 granite than upon gneiss or other foliated rocks. The reason of this 

 seems to be that in the bottom of the deep sea, where the eating away of 

 the rocks appears to have taken place, the granite was more susceptible 

 to disintegration than the others. Possibly this susceptibility may have 

 been increased by a somewhat higher temperature in the intrusive granite. 



"0^;e?^s" or Pits of Decay. — The northwestern portion of George island 

 and the greater part of the township of Rutherford, at the northwest ex- 

 tremity of Georgian bay, consist of red granite. Wherever the surface 

 of this rock has been protected from glacial action it is found to be eaten 

 into hollows in the form of round and sack-like pits and small caverns. 

 The latter generally occur on steep slopes or in perpendicular faces of the 

 rock, which, however, do not rise to any great height in this vicinity. 

 The floors of these caverns are usually flat, as they are formed by the 

 lower sides of horizontal joints, while the roofs are arched like ovens. 

 Whatever may have been the agency which excavated these ovens, it 

 always worked- inward from the granite face and U23ward from the floor 

 formed by a joint, the rock of the latter not being aff'ected, but remaining 

 as sound here as elsewhere along the joint-plane. 



One of these caverns, which occurs in the western part of Killarney 

 village, opposite to George island above mentioned, and which is shown 

 in figure 1 of plate 15, the reproduction of a photograph, has almost the 

 form and dimensions of the clay ovens used by the French Canadians 

 for baking bread, while other caverns in the neighborhood bear more or 

 less resemblance to this one; hence they have received the local name of 



