SUPERFICIAL GEOLOGY OF BRITISH COLUMBIA. 103 



clay lies directly on the rock surface ; in others the rich gold-bearing 

 gravels are quite distinct from and below this deposit. In Lightning 

 Creek, the material filling the bottom of the old stream-course, 

 often from 50 to 100 feet below the present brook, is a water-wasted 

 gravel, with little clay and no cementing matter, but much com- 

 pacted by pressure. This is richly auriferous, and has been 

 followed by extensive mining-operations. Above it, in the valley- 

 bottom and clothing the sides of the surrounding hills, is Boulder- 

 clay, which yields little gold ; and on this again are recent modified 

 deposits of gravel, over which the stream of to-day finds its course. 

 These last are generally also more or less auriferous, forming 

 " shallow diggings." 



Unmodified Drift. — The whole interior of British Columbia, up to 

 elevations of over 5000 feet, may be said to be more or less thickly 

 mantled with unmodified or scarcely stratified deposits, which I 

 shall refer to as Boulder-clay. Over considerable areas this material 

 is concealed beneath later accumulations, which form terraces and 

 low-level flats, in relation to present and former lake- and river- 

 valleys. There is a remarkable uniformity about these Boulder-clays 

 in every locality in which I have examined them. In many places 

 they form low rolling and broken hills, between the river-troughs, 

 above the level of the higher terraces. In this case they appear 

 sometimes to be spread in a comparatively thin layer over a rocky 

 substratum ; while in others they are of great depth, and, by the 

 irregularity of their arrangement, themselves produce many of the 

 minor features of the surface. They frequently show a tendency to 

 form more or less well-defined high-level plateaux, and are spread 

 almost universally over the elevated basaltic region of the interior, 

 in most places so uniformly, notwithstanding minor irregularities, 

 as to allow the underlying rock to be very seldom seen. The 

 Boulder-clay is quite typically developed on the basaltic plateau 

 crossed by the main waggon-road between Clinton and Bridge Creek. 

 This plain, isolated by deep river-valleys, is traversed by the road 

 for a distance of about thirty-five miles, and has an elevation of 

 from 3900 to 4200 feet. The Boulder-clay may be described as 

 consisting of a paste of hard clay, always more or less arenaceous, 

 and generally with a very considerable proportion of fine sandy 

 material, through which stones of all sizes are irregularly scattered. 

 Its usual colour varies from light-brown to pale greyish-brown and 

 fawn-colour, but in freshly exposed sections is sometimes bluish-grey. 

 It very often, over extensive regions, forms the soil in which the trees 

 are rooted, without the intervention of any modified material, and 

 is frequently so arranged as to surround larger or smaller depressions 

 which hold lakes and swamps. The greater part of the stones and 

 boulders contained in the clay or scattered over its surface, are 

 almost invariably rounded and waterworn, and sometimes very 

 perfectly so ; yet a proportion showing distinct glacial striation, and 

 some glaciated fragments worn flat on one or more sides, may always 

 be found. The quantity of distinctly glaciated material varies 

 much with locality, and, no doubt, depends in part on the nature of 



