PHYSICAL AND CLIMATAL CONDITIONS. 133 



questions is, at what stage of elevation or depression was 

 it produced. The second is that we must not infer that 

 the elevations and depressions were necessarily uniform 

 locally, but that they were different in amount in different 

 places, and that elevation of mountainous regions often 

 coincided with depression of plains, and vice versa. The 

 observations of Upham and Spencer as to what has been 

 termed " warping " illustrate this, and it has been finally 

 established by the work of Dr. G. M. Dawson on the 

 Cordilleran glacier of the west already noticed. 



It is not my purpose here to discuss the causes of the 

 elevations and depressions of the continents in the later 

 tertiary time. The attempt to account primarily for 

 depression or elevation of the land or the ocean level by 

 the accumulation of ice on the land is futile, since that 

 accumulation itself must have depended largely on the 

 changes of elevation and of consequent distribution of 

 land and water. Primarily, the great elevation of the 

 land must have been caused by the slow depression of J-he 

 ocean bed in the intervals between those local foldings of 

 the crust which result from and relieve such depression. 

 That some later or secondary portion of the local differen- 

 tial depression of mountain regions may have been caused 

 by the great weight of ice heaped on them is probable ; 

 but it is evident that this effect is quite inconsistent with 

 the idea of wide-spread continental glaciers. 



Students of glacial phenomena are no doubt right in 

 directing attention to the great sensitiveness of the crust 

 of the earth to pressure. The phenomena of river deltas 

 and of such great thicknesses of sediment as those of the 

 coal-formation, shows, as I have elsewhere often argued, 

 that every foot of sediment placed on any part of the 

 earth's crust must produce a corresponding depression 



