NATIONAL, ACADEMY BIOGRAPHICAL MEMOIRS VOL. VIII 



plemented in the "Physiographic Regions" by the statement 

 that the Rocky Mountains terminate in northern New Mexico, 

 where the Basin Ranges stretch far southeastward to meet 

 the southwestern border of the Great Plains. Subordinate 

 changes in Powell's boundaries and subdivisions may of 

 course be made, as in the interpolation of the group of domed 

 mountains in western North Carolina between the Piedmont 

 plateau and the Appalachian ridges, or in the separation of the 

 highlands of northern Minnesota and upper Michigan from 

 the Lake plains farther southeast ; but in the main the demar- 

 cation of the provinces here indicated in text and map consti- 

 tutes a permanent advance in American physiography^ How 

 singular that a practiced observer, keen enough to see that the 

 Rocky Mountains end southward in New Mexico, should not 

 have, as a writer for teachers, moderated -the hyperbolic pero- 

 ration which in the last lines of this essay described the Cali- 

 fornia coast ranges as a province /^ where the balm of the 

 tropics bathes the winter with verdure, and boreal zones boon 

 the summer with zephyrs K^ 



It was in connection with the explanatory or rational de- 

 scription of land forms in terms of their past history as de- 

 pendent on underground rock-structure and external erosive 

 processes, that Powell ingeniously applied his analytical 

 method in a reversed direction, as if confident that a good 

 rule must work both ways, for he frequently inferred the past 

 history of a district from its present form. The reading 

 of past history from depositional records had long been a 

 standard method in geology; reading past history from ero- 

 sional records was a novelty. This is well illustrated in his 

 conclusion that while each Basin Range "is but a small resid- 

 uary fragment of the great inclined block" from which it has 

 been carved, yet when compared to the Kaibab or the Uinta 

 "the erosion of the Basin Range ridges sinks into insignifi- 

 cance;" hence "we are forced to conclusion that the condi- 

 tions for great erosion now found in the Basin Ranges have 

 existed but for a short period" (Uinta, 33, 34). This prin- 

 ciple has had wide application in later years. 



