636 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



hard, comparatively homogeneous core covered with a relatively thin veneer of 

 already somewhat eroded, unmet amorphosed 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 unmetamor- 

 phosed 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 canyons 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 comparative homogeneity of the core- 

 rocks. For the same reasons, accordance 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 simul- 

 taneous accordance with the summit levels of the metamorphic mountains, except 

 for causes other than the two just described. As the composite 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. 



13. The influence of local glaciation on summit altitudes. — Hitherto no 

 detailed distinction has been necessary among the varied phases of erosion. It 

 may now be noted that the work of high-level glaciers, if long continued, tends 

 on the whole to produce summit-level accordance. In each glacier there are two 

 loci of maximum erosion; one at the head of the glacier where the great 

 bergschrund separates the ice from the solid rock of the head-wall; the other 

 beneath the central zone of the glacier itself some distance upstream from the 

 foot of the glacier. One result, noteworthy in the present connection, is to drive 

 the headwall of the growing cirque farther and farther into the mountain. In 

 the nature of the case, it will be the higher peaks which are most vigorously 

 attacked. From every side, it may be, comes the attack on the massif which, 

 for any cause, specially projects above the general level of the range. Owing 

 to the rapidity of the ice-erosion, that summit must tend to fall and reach some- 

 thing like accordance with its formerly lower, unglaciated or but lightly glaciated 

 neighbours. 



We have seen that all across the Cordillera the highest peaks and ridges long 

 suffered specially powerful attack, as they alone stood high enough to wear the 

 fatal belts of bergschrund. During the ice period, they were nunataks and lost 

 substance like nunataks; the loftiest peaks losing most, the lower ones with less 

 linear extent of bergschrund, losing proportionately less. Peaks and ridges not 

 penetrating the general surface of the Cordilleran glacier lost nothing by special 

 schrund-line attack.* 



* Compare the views of W. D. Johnson and G. K. Gilbert, as announced in the 

 Journal of Geology, Dec., 1904. The special glacial attack on the highest summit of 

 the Big Horn Range (Cloud Peak) is excellently illustrated in the well-known paper 

 by Matthes. Twenty-first Annual Report, U.S. Geol. Survey, Part II, 1899-1900, Plate 

 XXIII. 



