SUPERFICIAL GEOLOGY OF BRITISH COLUMBIA. 107 



sent smaller basins were united, which may possibly have allowed 

 the access of the sea to some extent. They have the appearance of 

 being the deposit from water charged with fine glacier-mud, or flour 

 of rock ; while the presence of glaciated boulders proves either that 

 some of the glaciers yet extended far enough from their sources to 

 reach the waters of the lake, or that heavy coast-ice existed. 



In approaching the mouth of the Nechacco, gravelly deposits of 

 great thickness are found, forming exposures in some banks over 

 200 feet in height. The gravels alternate irregularly with sands ; 

 and the whole is quite frequently false-bedded on such a scale as to 

 imply very strong current-action. The material of the gravels here 

 too ceases to bear so close a relation to the country rock, becoming 

 mixed with a considerable proportion of quartzite shingle, the origin 

 of which will be referred to hereafter. These sandy and gravelly beds 

 appear in the main to intervene between the Boulder-clay and the 

 White Silts, but are most closely related to the latter. They may indi- 

 cate the course of stronger currents coming from the north with much 

 shingle-laden coast-ice, the upper part of the Nechacco basin being 

 at this time a great sheltered bay in which the more typically deve- 

 loped White Silts were being formed. With the exception of the 

 Fraser valley, which may have been filled with the earlier drift de- 

 posits, the lowest gap in the southern rim of the White-Silt basin is 

 found near the southern sources of the Chilacco, at an elevation of 

 2660 feet ; and here a wide belt of country shows coarse and fine 

 superficial sandy deposits, with little gravel, forming low mounds 

 and ridges, which evidently owe their forms to moderately powerful 

 current-action, and may show the southern continuation of the 

 current above indicated. 



Shore-lines, Terraces and Benches.— The interior of British 

 Columbia shows water-marks in different stages of preservation, from 

 a height of 5270 feet down to the present sea-level. Some facts 

 bearing on this division of the subject have already been given in 

 connexion with other matters ; it will be necessary here to give 

 only a brief review of the more important features, mentioning a 

 little more fully a few of the most remarkable. It is necessary, 

 though sometimes difficult, to distinguish as far as possible between 

 shore-lines of the sea or former great lakes, and terraces which are 

 due to the gradual lowering by erosion of river-beds in their valleys, 

 which bear a quite different significance. 



The highest observed beach is that of which the elevation (5270 

 feet*) has already been given as probably the upward limit of the 

 Boulder-clay ; it was found on the northern slope of Il-ga-chuz 

 Mountain. The undulating and more or less broken plateau stretch- 

 ing eastward from this mountain, with a general elevation of about 

 4500 feet, appears to owe the form of its surface in great part to the 

 arrangement of drift upon it, and shows much foreign material. On 



* This height may be regarded as fixed with some accuracy, the figures 

 being the mean obtained from two barometric observations made on the terrace, 

 and simultaneously at a neighbouring station, of which the elevation had been 

 instrumentally fixed. 



