Ii6 REGINALD A. DALY 



formations is more resistant to the weather than an adjacent for- 

 mation. That experience is common in the alpine districts where 

 accordance of summit levels has been described. The implication 

 is that the real differences in power to resist attack are of a low order 

 among the rocks of these districts. The writer has often been struck 

 with this fact in the course of field-work in the Coast Range and 

 Selkirks of British Columbia. There, as generally, the phenome- 

 non must be attributed mainly to wholesale metamorphism. This 

 relative homogeneity among the rocks must be regarded as playing 

 an important part in the preservation of summit-level accordance. 

 Whether inherited or not, accordance will be clearly favored by 

 homogeneity. 



Secondly, the original upper surface of the zone of intense meta- 

 morphism may be conceived as much less uneven than the outer 

 surface of the original range. Mr. Van Hise has shown that pressure 

 is the principal control in the metamorphism of the zone of rock- 

 flow. 1 In the present case, pressure is applied by tangential force 

 and by the weight of individual massifs. The former is in dominant 

 control, as shown by the generally steep dips of planes of schistosity. 

 The lines of force in the tangential pressure are, on the average, 

 not far from horizontal. In the later stages of the period of plication 

 the master-lines of that force pass beneath structural depressions in 

 the range. During the same time constructional massifs will largely 

 escape the maximum squeezing which affects their bases. The 

 weight of each massif will, however, cause the metamorphism to 

 extend upward locally in some degree. The upper surface of the 

 metamorphic core of the massif will have a flatter and, probably, 

 a more regular profile than the rugged land surface above. The 

 composition of the two forces due to weight and tangential pressure 

 should, then, tend to produce a relatively simple upper surface for 

 the whole zone of metamorphism. The surface will be a flat arch 

 as a whole, but locally bearing subordinate domes of low curvature. 

 Along with these subordinate domes must be others of similar low 

 curvature due to the thermal metamorphism of batholiths. 



Many of the great intrusive bodies of alpine ranges had originally 

 themselves a demonstrably dome-like form with broad, flatfish tops. 



1 Monograph XLVII, U. S. Geological Survey (1904), p. 43. 



