274 



G. M. DAWSON ON THE SUPEEFICIAL GEOLOGY OE 



from a distance, those on the lower part of the slopes show 

 flattened outlines, while those higher up are more abruptly rounded 

 and have not been so thoroughly ground down. 



The general statements made in a former communication, in 

 reference to the covering of Boulder-clay or unmodified drift spread 

 over the entire area of the interior plateau, are borne out in the region 

 now more particularly in question. Prom the rearrangement of 

 this material the great systems of terraces subsequently mentioned 

 have been formed. 



Details need not be given of the evidence in striation and rock- 

 polishing of the existence of glaciers radiating from the various 

 mountain-systems, though it may be mentioned that some of these 

 seem to have had a very great extension down the lower valleys. 



In this southern portion of the interior plateau, terraces are 

 exhibited on a scale scarcely equalled elsewhere. They border the 

 river-valleys, and at greater elevations are found expanding beyond 

 these and attached to the higher parts of the plateau and mountains. 

 None has yet been found here, however, equal in height to that 

 previously described on Il-ga-chuz Mountain in the north at 5270 

 feet above the sea. Many of the terraces and " benches " of the 

 valleys may be the result of the gradual cutting-down of the river- 

 course in the hollow previously filled with glacial debris ; but for others, 

 including more particularly those of the higher levels, no explanation 

 short of the complete flooding of the plateau-region will suffice. 

 Knowing therefore that the water must have stood successively at 

 every lower level, it is of comparatively little importance that in the 

 case of some of the lower terraces it becomes impossible to determine 

 whether they belong to this period of the retreating waters or to a sub- 

 sequent river-erosion. 



In this region the terraces frequently surpass 3000 feet in eleva- 

 tion above the sea-level. The more prominent of those seen on the 

 southward slope of Iron Mountain may be taken as an example 

 of the arrangement of these old water-marks. These terraces are as 

 follows, the approximate heights being given in feet — 2386, 3063, 

 3392, 3611, 3715. It is frequently observed, however, that the 

 occurrence of a terrace at any particular level is merely a matter 

 of local circumstance, probably dependent on the supply of material 

 and other such causes ; and in different places not very remote the 

 the scale of terraces often differs. This is illustrated on Okanagan 

 Mountain, situated east of the lake of the same name. On the 

 south side of this elevation the principal terraces were baro- 

 metrically determined as follows— 1862, 2042, 2141, 2645, 2800, 

 2839 feet ; on the northern slope six principal terraces were again 

 observed, as follows— 1451, 1579, 1962, 2452, 2553, 2879 feet. 



The wide trough-like valleys which traverse the plateau are, over 

 a considerable portion of its extent in the southern part of the province, 

 partly filled with a deposit of white silt or loess-like material com- 

 parable with that described under the same name in the JNechaeco 

 basin to the north *. It is, however, unconnected with the latter, 

 * Quart. Journ. Greol. Soc. vol. xxsiv. p. 105. 



