SUMMIT LEVELS AMONG ALPINE MOUNTAINS 113 



sives, but also over much wider areas about the downwardly expand- 

 ing bases of the batholiths, the heat of the intrusions still further 

 increases the plasticity of the basement on which the mountains are 

 growing. The weakness of the underpinning is further manifest in 

 the case of such ranges as the Cascades or the Coast Range of British 

 Columbia, so largely formed of granitic magma injected in a highly 

 plastic, if not thoroughly fluid, state during or just after the last great 

 period of plication in those ranges. 



The conclusion seems unavoidable that the tendency of tangential 

 force to erect orogenic blocks projecting much higher into the air than 

 Mount Everest itself is operative only up to a certain critical point. 

 Beyond that point the increasing weight of the growing block and the 

 increasing plasticity of its basement call in another kind of movement 

 due to the gravitative downcrushing of the block. As a whole, or in 

 fragments separated from each other by normal faults, the block will 

 assume a shape and position suitable to static equilibrium for the 

 whole range. The range might conceivably find that equilibrium 

 when the entire uplift has attained the form of an elongated arch 

 accidented by already roughly accordant mountain summits. At 

 any rate, subequality of height might characterize large areas. 



This whole phase of gravitative adjustment forms a problem 

 clearly indeterminate in the present state of geological physics. 

 Critical laboratory experiments have yet to be devised, and careful, 

 special field-work devoted to the problem, before it can attain even 

 an approximate solution. So far as it goes, however, gravitative 

 adjustment of the kind just described aids all the other processes 

 tending toward summit-level accordance. 



3. Hypothesis of original rough accordance, due to differential 

 erosion during the period of alpine plication. — Co-operating with 

 isostatic adjustment is the effect of the special erosive attack on each 

 rising block from the moment it once begins to dominate its surround- 

 ings. On the average, the forces of weather and waste are most 

 destructive on the summits of this time, as they shall be through all 

 the subsequent history of the range as an alpine relief. Denudation 

 is in some direct ratio to the height of uplift. Higher summits are 

 thereby reduced, while lower ones are still growing under the stress of 

 mountain-building. How far erosion thus checks the upward growth 



