•RIVER AND LAKE CHANNELS DUE TO ROCK DECAY. 865 



that it has given rise to the main channel of the stream. The gneiss has 

 been altered and shattered for some distance from the walls of the dike, 

 and this has also suffered decay and erosion and now forms a parallel 

 channel on either side of the central one. Between them the finer 

 grained, hard and undecaj^ed greenstone constituting the outer portions 

 of the dike rises up in the shape of ridges and chains of islands, so that 

 the river flows as a main central channel more or less separated from 

 the smaller lateral ones. 



Lake Temiscaming, on the Ottawa, like the Montreal river, which 

 enters its southern extremit}', appears to follow the course of a great 

 deca3''ed dike or set of parallel dikes. It lies in a narrow depression 

 which cuts across the general strike of the Archean rocks of the region, 

 and its surface is probably 500 feet below the average elevation of the 

 surrounding countr3^ Its width is only from a mile to two miles, and it 

 has a length of thirty-five miles, but the channel is continued in Deep 

 river, the name given to the section of the Ottawa immediately below it. 

 Temiscaming is an Ojibwe word meaning " deep lake," and it is well 

 named, for in one part it measures, according to Mr A. E. Barlow, 1,800 

 feet in depth. The bottom is covered very unequally with drift, which 

 has been pushed into it by glaciers at different times, and the deepest 

 part of the bottom ma}^ still be a considerable height above the sound 

 greenstone below this filling. If we suppose this height to be 300 feet, 

 then we liave a total of 2,600 feet as representing the average height of 

 the surrounding country above the bottom of the excavation. The sur- 

 face of the lake is 612 feet above the sea, so that the bottom of the water 

 is about 1,200 feet, and that of the whole rock excavation may be 1,500 

 feet or mm-e below this level. This may, therefore, be regarded as a very 

 deep cutting across a country, the general aspect of which is that of a 

 mammillated plateau with few great inequalities. 



The islands toward tlie north end of the lake consist of limestone of 

 the Niagara formation, which also rises in cliffs at its extremity and 

 extends northward a considera])le distance beyond the lake in the same 

 orographic depression, while the Upper Ottawa river falls over the side 

 of this valley by a rapid descent from the plateau to the eastward. 



This valley tlierefore existed before the date of the Niagara formation, 

 and it is ])roba})le that under the limestone just mentioned may be found 

 older members of the Silurian system. Greenstone dikes which so fre- 

 quently cut the Archean rocks of northern Canada have never been 

 found to traverse the overlying Silurian, and we are therefore warranted 

 in supposing that the valleys which mark the courses of the decayed 

 dikes among the former class of rocks were mostly formed before the 

 deposition of the Paleozoic strata. 



LI— Bull. Geol. Soc. Am., Vol. 5, 1893. 



