SUMMIT LEVELS AMONG ALPINE MOUNTAINS 115 



II. THE SPONTANEOUS DEVELOPMENT OF SUMMIT-LEVEL ACCORDANCE 



i. Spontaneous development by isostatic adjustment. — The last 

 paroxysm of crumple and upthrust in the young alpine range has 

 occurred. Henceforth its forms are to be determined chiefly by 

 erosive processes — yet not altogether so. Several authors have 

 suggested that the leveling influence of gravity is not only manifest 

 in the piecemeal carriage of rock fragments out to the piedmonts, 

 or finally to the sea; but that also the very accordance of summit 

 levels is in large part related to gravitative adjustment on a large 

 scale. 1 Where, for any cause or causes, denudation significantly 

 lowers a localized area of the range faster than neighboring areas of 

 the same altitude, the former area will tend to rise, the surrounding 

 region to sink, so as to reproduce conditions of equilibrium in the 

 range. This view entails belief once again in the principle of isos- 

 tasy. It must be admitted that the ground has only been broken 

 in the important field of inquiry as to crustal sensitiveness. The 

 harvest of field and experimental observations has not yet been 

 reaped in volume sufficient to enrich geological science with definite 

 knowledge on the matter. But such facts as the apparent isostatic 

 recoils of the earth's crust after the melting away of the Scandinavian, 

 Labrador, and British Columbia Cordilleran ice-caps, and the notable 

 increase of dips at the feet of the High Plateaus of Utah and Arizona, 

 as described by Major Dutton 2 , are among those already recorded 

 in favor of a sympathetic entertaining of the isostatic doctrine. The 

 appeal to the principle in the present case is all the more worthy 

 because of the long continuance of the special plasticity belonging 

 to the very slowly cooling basement of a recently folded alpine range. 



2. Met amor ph ism and igneous intrusion in relation to the degra- 

 dation 0} mountains. — It is a truism that the rocks of any alpine range 

 vary enormously in composition and structure. It is quite as true 

 that their resistance to weathering and wasting is far less variable. 

 In hundreds of square miles together, the geologist may map gneisses, 

 schists, granites, diorites, marbles, quartzites, or ancient lavas, 

 several or all of these occurring in great masses, and yet he may not 

 be able to ascertain by manifest field evidence that any one of the 



1 See discussion in Penck's Morphologie der Erdoberfldche. 



2 Monograph II, U. S. Geological Survey (1882), p. 47. 



