634 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



increasing weight of the growing block and the increasing plasticity of its 

 basement call in another kind of movement due to the gravitative downcrush- 

 ing 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 equili- 

 brium 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. 



In this connection we may note the prevalence of normal faults in the 

 Purcell mountain system, which is one of the most noteworthy of all the ranges 

 in showing summit-level accordance on a large scale. As stated in the chapter 

 on the stratigraphy and structure of the Purcells, most of these faults have 

 probably been developed as a result of crustal adjustments following the severe 

 upturning of the Laramide revolution. 



3. Hypothesis of original rough accordance, due to differential erosion dur- 

 ing 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 surroundings. 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 of the rising massifs probably cannot 

 be measured, but such differential destruction must develop still further the 

 rough summit-level accordance already in part established by isostatic adjustment. 

 ■ The downcrushing of higher, heavier blocks with the simultaneous rise of 

 their lower, lighter neighbours, coupled with the likewise simultaneous, specially 

 rapid loss of substance on the higher summits, form a compound process leading 

 toward a single, relatively simple result. In both the architecture and the sculp- 

 ture of her alpine temple, Nature decrees that its new domes and minarets shall 

 not be indefinitely varied in height. Such accordance as they have among them- 

 selves will be preserved and accentuated as her chisels fashion new details on the 

 .building. The accordance of the present time in any alpine range is in part 

 inherited from what, in this paper, has been called the ' original ' form of the 

 range. The original form meant a first approximation to the result; the later, 

 spontaneous modification of that form means a second approximation to perfect 

 accordance. 



In passing to an analysis of erosion events following the epoch of folding, we 

 are, therefore, illustrating the cumulative force of both the hypotheses so far 



