SUPERFICIAL GEOLOGY OF BRITISH COLUMBIA, 119 



It is, however, possible tbat in one other manner a great glacier, 

 moving from north to south, may have filled the central plateau. 

 Owing to the warm water of the great Japan current, with the 

 prevailing westerly and south-westerly winds and mountainous 

 character of the coast, the annual precipitation of moisture is very 

 great, especially to the north. At Sitka (southern end of Alaska) 

 the average annual depth of rain and melted snow (from sixteen 

 years' observations) is 82*66 inches, or within a fraction of seven 

 feet ; while the average number of days on which rain, snow, or 

 hail fell, or heavy fogs prevailed, is two hundred and forty-five, or 

 two days out of three. It may be supposed that, under certain 

 not improbable combinations of conditions, the mountainous country 

 to the north, above referred to, became preeminently the condenser 

 of the Northern Pacific, and, from the mere accumulation of snow 

 and ice, the focus of glacier- action and point of radiation of great 

 glaciers. If the central plateau was ever filled thus by a great 

 glacier-mass, the ice must have poured southward through the 

 gaps on the 49th parallel, and westward across the Coast range, 

 in a manner similar to that in which the ice supposed by Professor 

 Geikie to have filled the Gulf of Bothnia must have crossed the 

 Scandinavian peninsula*. 



If the first glaciation of the central plateau is due to the action 

 of glaciers as such, we should, however, expect to find remnants at 

 least of deposits like those elsewhere ascribed to such gigantic ice- 

 sheets, and not precisely resembling that noticed on a former page 

 under the name of Boulder-clay. If these exist, they have not fallen 

 under my observation ; and in any case it appears necessary to call 

 in the action of water with floating ice to account for the formation 

 of the Boulder- clay, with its rounded pebbles and irregularly distri- 

 buted erratics. It cannot have been laid down by glacier-ice ; for 

 it is difficult to imagine the formation of material which is found 

 not only over plains but on exposed hill-slopes and summits, beneath 

 a great glacier which we find in other places engaged in scooping 

 rock-basins in the bottoms of valleys. It rests immediately on the 

 well preserved glaciated surfaces, and, on the above suppositions of 

 an extension of the ice- cap, or great central-plateau glacier, may 

 have been formed during the gradual retreat northward of the 

 decaying front of the ice while the country was submerged to a 

 depth of over 5000 feet, either by access of the sea, due to general 

 depression, or by the formation of a great lake covering the plateau 

 region, the passes of the Coast range — those to the south, and those 

 of the Rocky Mountains to the east, being still blocked by local 

 accumulations of glacier ice. The movement of icebergs would 

 explain the irregular distribution of the foreign mixed materials of 

 the deposit. Water-action, sufficient fco account for the rounding of 

 the pebbles, may have occurred ; and the gradual diminution of the 

 glaciers in the various mountain-ranges may be supposed to have 

 allowed the slow drainage of the lake, and given rise to the great 

 systems of terraces. 



* Great Ice Age, p. 404, 



