250 DAVIS — SYSTEMATIC GEOGRAPHY. [Aprils, 



14. Distinction of Geography fro7n Geology. — If the explanatory 

 method is adopted as appropriate for the physiographic description 

 of meandering valleys in the narrow scroll stage, the same method 

 should be adopted for all other stages of valley carving and for all 

 other land forms as well. The orderly action of natural processes 

 through a portion of past time is implied in such a phrase as ^* the 

 narrow scroll stage," and it is similarly implied in saying that 

 the AUeghenies of Pennsylvania are of corrugated mountainous 

 structure, essentially baseleveled in a former cycle ; then broadly 

 elevated and thus standing long enough for the weaker strata to be 

 etched out as lowlands, leaving the harder strata to stand up as 

 even-crested ridges \ and then again moderately elevated long 

 enough ago for the valley lowlands to have now reached a subma- 

 ture stage of dissection. The descriptions of the Susquehanna val- 

 ley and of the Pennsylvania AUeghenies differ in the quantity of 

 past process and of past time involved ; but such a difference is 

 only of degree, not of kind. If all the stages of development 

 through which the Pennsylvania AUeghenies have passed are traced 

 out for their own sake, as much attention being given to one 

 stage as to another, then the study is truly geological. If the 

 changes of the past are introduced only in so far as they illuminate 

 the present, and with no other object than to secure such illumina- 

 tion, then the study is geographical. It would be as much a mis- 

 take to regard such study as geological as it would be to say that a 

 chemist is studying physics when he uses a balance to weigh a pre- 

 cipitate, or that he is studying mathematics when he calculates 

 atomic weights. He is truly enough for the time employing physical 

 and mathematical methods, but he is studying chemistry. It 

 would be no more just to regard the explanatory description of 

 flood plains as belonging under geology because it has to deal with 

 past time, than to treat it as belonging to the study of physics be- 

 cause it involves the application of physical principles in the flow of 

 a stream, in the corrosion of its bed and banks and in the trans- 

 portation and deposition of detritus ; and surely it would be no more 

 appropriate to regard such a study of flood plains as a part of 

 physics than it would be to take away the spectroscopic study of 

 the stars from astronomy. 



15. Dangers of Explanatory Description. — It is sometimes ob- 

 jected that the explanatory method of description is dangerous, be- 

 cause the observer who seeks to add explanation to observation 



