192 THE ICE AGE IN NORTH AMERICA. 



ander Archipelago, fill the space occupied by the Strait of 

 Georgia between Vancouver Island and the mainland, and 

 cover with ice the whole valley between Mount Tacoma and 

 the Olympian Mountains, where now we find the vast mo- 

 raine deposits of the islands and shores of Puget Sound. 



A simple calculation impresses one with a sense of the 

 unstable equilibrium of the forces leading to the increase or 

 diminution of a glacier. We estimated that 77,000,000,000 

 cubic feet of ice annually pass through the gorge at the head 

 of Muir Inlet, and that the area of the ice-field supplying 

 this stream is about twelve hundred square miles. The total 

 amount of ice entering the inlet, therefore, is only equivalent 

 to about two feet of ice over the field of supply. If from 

 any cause two feet more of ice should annually accumulate 

 over this area, or two feet less should annually melt away, 

 the amount of ice compelled to go through the gorge would 

 be doubled, and this would doubtless fill up the whole inlet 

 and bay to the south. When we reflect that, according to 

 Newcomb,* the average amount of ice which would be 

 melted by the sun over the whole earth is something more 

 than a hundred, feet a year, and that, therefore, a change in 

 intensity amounting to only one fiftieth of that exhibited 

 by the present meteorological forces would produce the re- 

 sults just mentioned, we can readily believe that oscillations 

 in such a great glacier may be frequent in occurrence and of 

 great magnitude. 



Southward, in Oregon, the Willamette Valley was filled 

 in a similar manner by an extension of the glaciers still lin- 

 gering on the flanks of Mounts Hood and Shasta. The 

 absence of drift on the southern shore of Vancouver Island 

 seems to point to a termination of the southerly movement 

 from Alaska in the Strait of Juan de Fuca, where perhaps 

 the confluent streams turned westward, and sent off vast 

 drift-laden icebergs to the sea. Mount Baker, immediately 

 to the east of this point, upward of ten thousand feet high, 



* " Popular Astronomy," p. 247. 



