AVERAGE REGIONAL SLOPE 43 



OBJECTIONS TO A SUBDIVISION BY AVERAGE REGIONAL SLOPES 



The objections which may be urged to a subdivision of this 

 kind are: (a) that it is an arbitrary one; (b) that the slopes on a 

 land form vary widely, and two observers may come to different 

 conclusions regarding it; (c) that it requires a greater amount of 

 detailed field and office work than is necessary when the average 

 slopes are not taken account of. 



a) The subdivision is arbitrary, for as far as we know there is 

 no distinct change in the cycle at either of the two limiting points 

 of 2 and 3 per cent, which is placed on the two type forms proposed 

 here. Moreover, it is probable that there are old erosion surfaces 

 to represent all stages of average regional slope from 6 per cent to 

 less than one-tenth of 1 per cent, and there may be as many examples 

 lying between the two types proposed as within the limits of the 

 peneplain type. 



A parallel might be drawn between the classification of igneous 

 rocks and the subdivision proposed here. In 1886 or thereabouts, 

 when geologists of the United States Geological Survey found large 

 areas in the Sierra Nevada Mountains underlain by intrusive 

 masses of approximately similar composition lying between the 

 quartz-diorite and granite families, they suggested the name 

 granodiorite for them. Thirteen years later Lindgren 1 proposed 

 definite limits for the "granodiorite" family in regard to both its 

 chemical and mineralogical composition. The bulk of the rock 

 masses referred to in the Sierras, and later found to occupy great 

 areas in Canada, fall within the limits proposed by him. His 

 quantitative restriction of the term granodiorite is therefore justi- 

 fied, for it represents a natural group of rocks. The term gains 

 stability, moreover, because of the occurrence of this group within 

 a definite and accessible region. 



This definition of granodiorite is of great value to petrographers 

 because it furnishes a clear-cut standard of comparison and datum 

 point within the scheme of classification. The occurrence of rocks 

 intermediate in composition between granodiorite and granite on 

 the one hand, or granodiorite and quartz-diorite on the other, does 



1 Waldemar Lindgren, "Granodiorite and Other Intermediate Rocks," Am. Jour. 

 Sci., IX (1900), 269-82. 



