570 THE ICE AGE IN NORTH AMERICA. 



"Likewise, concerning the glaciation of Europe, we find 

 that in Wales and in Yorkshire, England, the amount of 

 denudation of limestone rocks on which bowlders lie has 

 been regarded by Mr. Mackintosh* as a proof that a period 

 of not more than six thousand years has elapsed since the 

 bowlders were left in their positions. The vertical extent 

 of this denudation, averaging about six inches, is nearly the 

 same with that observed in the southwest part of the Province 

 of Quebec by Sir William Logan and Dr. Robert Bell, where 

 veins of quartz marked with glacial striaB stand out at 

 various heights not exceeding one foot above the weathered 

 surface of the enclosing limestone. 



As illustrating how little we know about the causes which 

 produce the variations in snow fall, even from year to year, 

 and render it impossible to form trustworthy a priori opinions 

 concerning the proximity of the causes which are capable of 

 producing glacial conditions, Mr. Becker writes that in 

 1890 the " snowfall in the Sierra was exceptionally large, 

 about two and one-fourth times the average precipitation 

 having fallen. Much of this snow remained unmelted through 

 the season, and when I left the mountains, on October 1, 

 there were still thousands of snowbanks where in ordinary 

 seasons none remains even far earlier in the season. Many 

 of these banks were also of great depths, say 100 feet, more 

 or less. It is clear, therefore, that were this and succeeding 

 winters to be as wet as the last, the range would show glaciers 

 in great numbers, much as the Alps now do; in short, tho 

 glacial period of the Sierra would recur in a moderate way. 

 Now, no one doubts that there was some cause for the unusual 

 snowfall of 1889-90 but no one has any suspicion what it 

 was. No sensible change in cosmical or terrestrial condi- 

 tions has occurred, the weather of the world at large was not 

 remarkable, and, excepting as to precipitation, the year was 

 not extraordinary even in California." 



* "Quarterly Journal of the Geological Society," vol. xxxix, pp. 

 67-69; vol. xlii, pp. 527-539. 



