SUPERFICIAL GEOLOGY OF BRITISH COLUMBIA. 105 



Modified Drift. — As already mentioned, extensive areas of the 

 boulder-clays are entirely concealed by newer overlying deposits. 

 It may be almost stated as a rule that, in regions lying lower than 

 3000 feet, the superficial materials are entirely of this more recent 

 date. These are often clearly traceable to ancient lakes, of which 

 they yet define the outlines, but in other cases are due merely to the 

 successive rearrangement of material by rivers slowly cutting down 

 their valleys. The extent of surface below the above-mentioned 

 limit in the southern part of British Columbia is comparatively 

 small ; and the more or less isolated later deposits present varying 

 local characters. To the north, however, the general lower elevation 

 of the country has allowed the formation of beds more wide-spread 

 and important. I have examined the southern portion only of this 

 lower area, the northern extension of which passes in a wide belt 

 along the Parsnip and through the Peace river-depressions in the 

 Eocky Mountains to the great Mackenzie river-basin*. 



The best sections of the deposits of this northern area are found 

 in the Lower Nechacco basin, between lat. 53° 30' and 54° 10', and 

 long. 123° and 124° 40'. They reach a height of about 2400 feet at the 

 edges of their basin, and where seen lowest (near Fort George) have 

 an elevation of 1900 feet. Their known area is about 1000 square 

 miles ; but they may extend far northward. They seem to represent 

 an interesting epoch of the Glacial period, and are composed of nearly 

 white, greyish or pale chocolate-grey arenaceous clay, divided by 

 distinct stratification-planes into layers an inch or two in thickness, 

 and very uniform in this respect. When dry the material is hard, 

 but not sufficiently homogeneous to break regardless of bedding- 

 planes. Under the microscope it is seen to be made up of very fine 

 angular quartzose particles, mingled with a little formless argillaceous 

 material. It is usually calcareous, and is filled in many sections 

 with calcareous nodules, generally lenticular, but often confluent, 

 forming grotesque aggregations. These are especially abundant on 

 the shore of Stuart Lake near Port St. James, and in some of the 

 JSTechacco-Biver sections. I have called these deposits the White 

 Silts. 



The beds are sometimes seen to be disturbed, and even more or less 

 contorted — an appearance which has, I think, often been produced 

 by "slides " in the banks of the river of comparatively recent date. 

 Contortion, however, sometimes seems to have been caused by the 

 contemporaneous action of floating ice, the presence of which is 

 proved by the occasional occurrence of large subangular boulders, 

 which generally lie in groups, and are often as much as two feet in 

 diameter. The portions of the formation holding many boulders 

 seldom show distinct bedding, though composed of material similar 

 to the rest, and not mixed with much gravel or beds of coarse de- 

 tritus intermediate in character between the fine matrix and the 

 erratics. In some cases the White Silts are found to have been de- 

 posited in gently inclined beds on preexisting sloping surfaces. They 



* Selwyn, Report of Progress Geol. Surv. of Canada, 1875-76. 



