396 W. M. DAVIS 



It may at first appear sufficient to say that high-standing desert 

 plains can have been made only in those regions which are now desert, 

 but this easy solution of the problem is hardly convincing. Climatic 

 changes are known to have occurred in the past, and inasmuch as 

 they did not all affect areas in a way that is sympathetic with the 

 present arrangement of the zones, the possibility of a former different 

 distribution of deserts from that which now occurs seems to be open. 

 Pleistocene climatic changes of the glacial kind were so modern and 

 short-lived that they have little bearing on the possibility of earlier 

 climatic changes of another order. The more ancient records of 

 glaciation are so distributed as to demand significant rearrangement 

 of the present climatic conditions. The existing deserts are, more- 

 over, of two kinds with respect to cause: some deserts, like those 

 of Africa and Australia, are arranged chiefly with respect to the 

 trade- wind belt; other deserts, like those of central Asia and the 

 southwestern United States, are dependent for the most part on the 

 extent and configuration of the surrounding highlands. When we 

 go back as far as Cretaceous time, it should only be by evidence 

 and not by assumption that we are led to regard a truncated upland 

 of that date as having been baseleveled during a cycle of normal 

 climate and afterward uplifted and dissected, instead of having 

 been leveled above baselevel during a cycle of arid climate, and 

 dissected in consequence of a change to a normal climate. A century 

 ago demonstrated movements of the earth's crust were matters of 

 astonishment; witness the surprise then felt at the discovery of 

 fossilized marine shells in some of the loftier Alpine ranges. Today 

 the crust is raised and lowered on the evidence of dissected pene- 

 plains, as in the Appalachian region, without exciting remark; it 

 is now the shifting of climatic conditions that would cause dissenting 

 surprise. It is difficult to determine how far such surprise is well 

 founded, and how far it simply reflects the fashion of our time. 

 Even if the climatic zones have always belted the earth as they do 

 now, the desert areas that depend on the configuration of land and 

 water, and of highlands and lowlands, have certainly varied through 

 the geological ages. It is therefore desirable, wherever the question 

 of "uplifted and dissected peneplains" is raised, to scrutinize it care- 

 fully, and to determine, if possible, whether it is really the attitude 



