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



of the much newer rocks were deposited. The quartzites appear to have 

 undergone no change in texture, structural attitude or even surface con- 

 tour since a period vastly antedating Silurian time. Such facts as these 

 indicate pretty clearly that the physical features of the older parts of 

 our continent had their origin at a very remote period. 



River and Lake Channels due to Rock Decay. 



The writer has elsewhere* shown that many of the long, straight val- 

 leys in the Archean regions of Canada, now often occupied hy straight 

 river stretches, by long, narrow lakes, and by inlets of the larger lakes, 

 are due to the decay and removal of wide greenstone dikes or of paral- 

 lel dikes, together with the belts of rock between them. When the 

 depressions along these decayed dikes are not overspread by water they 

 form valleys, more or less filled up with drift, so that the greenstone in the 

 bottom may be only exceptionally exposed to view. Copious springs 

 frequently issue from the drift in these valleys. 



The long, narrow and straight inlets of the northern part of Georgian 

 bay have had their origin along the courses of dikes of this class. The 

 continuation of each of these channels out into the bed of the lake in 

 front of it is distinctly marked by a straight line of deeper soundings, 

 having the same direction as the fiord itself. Collins, Key, Henvey and 

 Byng inlets, each about twelve miles long, but averaging only 200 or 300 

 yards in width, are good examples of these fiords. Among the more con- 

 spicuous of the long, narrow and nearly straight valle3^s of the Archean 

 region which have been formed in the manner just described may be 

 mentioned that of Onaping lake, 30 miles long, north of lake Huron ; 

 Long lake, 52 miles long, north of lake Superior, and Sepiwesk lake and 

 Nelson river below it, 96 miles long, north of lake Winnipeg. The green- 

 stone of the dike or dikes along the course of these channels may be seen 

 only on islets and points or in patches adhering to the country rock 

 on either side. In the case of Long lake, an immense greenstone dike 

 emerges from its southern extremity and runs into the hills in the same 

 bearing as the central line of the lake. 



The course of the Mattagami river, the central branch of the Moose, is 

 guided in its northward trend for a distance of 160 miles from the head of 

 lake Kenogamisse by a number of long dikes of greenstone from 200 to 

 1,000 feet wi.de, all having nearly the same direction.f In a typical sec- 

 tion of this part of the river we find that the central portion of the dike 

 is coarsely crystalline and more deeply eroded than at the sides and 



* Reports Geol. Survey Canada, 1870, p. 331 ; 1875, p. 315; 1878, p. 15 CO; Report Bureau of Mines 

 Ontario, 1891, p. 76; Bull. Geol. Soe. Am,, 1890, p. 300. 

 t Report of R. Bell : Geol. Survey of Canada for 1875, p. 315. 



