STRUCTURAL FEATURES OF THE FRE-CAMBRIAN 191 



of conglomerate. Only one outcrop of this was seen, about a mile 

 below the mouth of the Brock River. At this point the conglomer- 

 ate is very schistose. The pebbles are so numerous as almost to 

 exclude matrix, which is a rather impure sand. About 30 per cent 

 of the pebbles are hornblende granite; the remainder are various 

 basic rocks, now so sheared that their original composition is inde- 

 terminable, although it is clear that they are of several kinds. The 

 stresses to which the rock has been subjected have deformed the 

 pebbles according to their original hardness. The softer have been 



Brock ssrias 



lavas 



Geolqgical boundary 

 Oip and sirilee 



Fig. 8. — The Brock area 



converted into narrow strings several inches in length by a fraction 

 of an inch in width; harder ones have been flattened into lenses the 

 shape of which depends on the original hardness; while those of 

 granite, the most resistant, have been deformed by fracture rather 

 than by flow, and still retain much of their original shape. The 

 softer pebbles have flowed around the harder, accommodating them- 

 selves most closely to all their curves. Fig. 9 illustrates this con- 

 glomerate. The almost undef ormed granite pebbles are well shown ; 

 in some cases even the fractures in them, inclined at about 45° to 

 the planes of schistosity, may be seen. Some of the more promi- 

 nent pebbles of softer materials are also dearly visible, with the 

 manner in which they have flowed around the harder granites. 



