SUMMIT LEVELS AMONG ALPINE MOUNTAINS 117 



The foregoing statement of a difficult theme is brief, but it suffices 

 to suggest the bearing of metamorphism and intrusion on the ques- 

 tion of accordance. In what has been defined as its original state, 

 an alpine range was composed of a hard, comparatively homo- 

 geneous core covered with a relatively thin veneer of already some- 

 what eroded, unmetamorphosed rock. The core is to be conceived 

 as having an upper, limiting surface, with the form of a long, flat 

 arch bearing subsidiary, low, broad, boss-like arches and domes. 

 The erosion of the unmetamorphosed cover will go on apace. The 

 erosion of the core, the main mass of the range, will progress much 

 more slowly. Erosion may thus sweep away wide areas of the cover 

 before the individual mountains between canons sunk in the core 

 have suffered significant loss of height by denudation. In such areas 

 accordance of summit levels would henceforth be expected because 

 of the original flattish tops of the core, and because of the compara- 

 tive homogeneity of the core-rocks. For the same reasons, accord- 

 ance among the summits of mountains cut out of a granite batholith 

 would be expected. Where, however, the granite is distinctly harder 

 than the surrounding metamorphics, there would not be simultaneous 

 accordance with the summit levels of the metamorphic mountains, 

 except for causes other than the two just described. As the com- 

 posite explanation of accordance is further outlined, it will be seen 

 that such other causes may operate effectively in some cases. Yet 

 the common, special dominance of granite peaks in a truly alpine 

 range agrees as well with the composite explanation as it does with 

 their reference to the class of monadnocks on the peneplain theory. 



3. The influence of local glaciation on summit altitudes. — Hitherto 

 no detailed distinction has been necessary among the varied phases 

 of erosion. All sub-aerial agencies of destruction combine their 

 effects to establish so much of summit-level accordance as is due 

 to erosion with consequent isostatic adjustment. Each of the agencies 

 may take a part in the uncovering of the hard, metamorphic core 

 of the range. Throughout the entire history of the range, however, 

 special kinds and conditions of denudation independently do im- 

 portant shares of work in trimming the range to uniformity or accord- 

 ance of summit levels. To ascertain the value of their work we 

 must take the highland view. A few decades ago, when the power 



