650 ALFRED IV. G. WILSON 



an outlying monadnock ridge rising through the Paleozoic sedi- 

 ments, is comparable with these, and it is not impossible that its 

 height above the peneplain on which it stands, but whose surface 

 in this vicinity is obscured by the sediments, may be even greater. 



Another excellent example, and one of considerable historic 

 interest in connection with Logan's early work on the geology 

 of the Archaean, is Trembling Mountain, lying north of the 

 Ottawa River, and to the northwest of the city of Montreal, 

 The mountain is a domed monadnock rising over 800 feet above 

 the general level of the plateau, and about 2,380 feet above sea- 

 level. 



7. Lakes. — Reference has already been made to the fact, 

 characteristic of the whole area, that the majority of the minor 

 depressions on the surface of the peneplain form innumerable 

 rock-basins in which the water gathers to form large and small 

 island-dotted lakes. In addition to lakes of this class, which 

 are usually very shallow and with very intricate shore lines, we 

 occasionally find several other types. 



One of the commonest of these is longitudinal lakes occupy- 

 ing old valleys which are more or less blocked by debris or by 

 other geologic causes, in some cases presumably differential 

 uplift. There are other examples of long, narrow lakes whose 

 depressions are ascribed to the erosion of softer dikes. Over 

 fifty years ago Agassiz drew attention to the fact that many of 

 the bays and points on the shores of Lake Superior owed their 

 origin to the erosion of soft dike rocks. More recently Bell has 

 explained the origin of certain channels along the Lake Huron 

 shore, and the longitudinal basins of certain inland lakes north 

 of Lakes Huron and Superior, by a similar process. 



The most interesting of the lakes of the longitudinal type is 

 that of Temiscaming and the "Deep River" of the Ottawa 

 adjacent to it. Dr. Barlow's description of this lake has already 

 been mentioned, and his description of the character of the 

 depression which it occupies has been quoted above. Whether 

 the depression is of the graben type, somewhat similar to Lake 

 Tanganyika in central Africa, or whether it is to be ascribed to 

 the blocking or warping of a preglacial gorge is at present 



