98 GROVE KARL GILBERT— DAVIS CM " MOIBS [v i£xxt 



where evidence of glaciation is not ordinarily seen, I was impressed with the topographic types because they 

 differed from those that I was familiar with, and I was led to study the causes of their development and write 

 an analysis of land sculpture by weathering and streams. 



The increased understanding that he reached of glacial sculpture in his home district after 

 he had gained an understanding of the normal processes and products of land sculpture in the 

 West is alluded to in a later section. 



The famous chapter on "Land sculpture," which is expanded from a briefer statement of 

 the same problem in the essay on the " Colorado plateau province as a field for geological study," 

 above analyzed, offers an illustration of Gilbert's manner of thought that is both pleasing and 

 edifying; and it is particularly with reference to the revelation of his inner nature thus afforded 

 that the chapter is here reviewed. It exemplifies a principle which he announced later, that in 

 the teaching of a science more attention should be given to its philosophy than to its material 

 content; for having recognized that an understanding of sculpturing processes would lead to the 

 understanding of sculptured forms, he expounded the philosophy of the elementary processes 

 of erosion as well as philosophy of the evolution of land forms, and set forth both doctrines in 

 the most genial and competent manner, thus elevating them to the grade of serious studies. 

 For example, as to processes one may learn, under the heading of "Transportation and com- 

 minution," just why it is that a river which is carrying a maximum load of detritus of varied 

 texture will, when its velocity is diminished, not decrease its load by laying down a fraction of 

 detritus of all textures, but by laying down only the coarsest detritus; or, in more general terms, 

 why it is that when a river is accelerated it will successively select coarser and coarser detritus 

 to take up; while when it is retarded, it will select first the coarsest and then successively the 

 less and less coarse detritus to be laid down (107). Similarly, clear explanation is offered on 

 sufficient physical grounds for the fact that "in any river system which is fully supplied with 

 material for transportation [as a river system usually is at time of flood] and which has attained 

 a condition of equal action [at present often denoted by the term, grade, which Gilbert himself 

 later introduced], the declivity of the smaller streams is greater than that of the larger" (114). 

 These facts were, 50 years ago, already familiar enough as matters of observation, yet few 

 students of earth science then — -and perhaps now also, for that matter — concerned themselves 

 with a critical analysis of the causes of the facts. Hence, to find their causes logically analyzed 

 was a gratification to the mind of many a reader; and the gratification thus excited was soon 

 transmuted into gratitude toward the author of the analyses. 



It was the same with the discussion of land forms as the result of erosional or sculpturing 

 processes upon crustal masses. Many matters that were familiar enough as facts of occurrence 

 were systematically formulated, greatly to the advantage of the study of the physical geogra- 

 phy — or the "physiography," as it was later called — of the lands. Following Powell's lead in 

 his Colorado River report, it was shown in the first place that the rock structure of a land mass 

 is a fundamental factor in the determination of its sculptured form ; and here was given the 

 warrant, if indeed warrant were needed, for the introduction of "structure" as the first term 

 in the explanatory description of land forms by those physiographers to whom a rational 

 treatment of their subject is preferred over an empirical treatment. It was next shown that, 

 supplementary to the "law of structure" is the "law of divides," according to which a 

 homogeneous mass will come to have its steepest slopes at the stream headwaters and its gen- 

 tlest slopes at their mouths. Under these two laws, "the features of the earth are 



carved The distribution of hard and soft rocks or the geological structure, and the 



distribution of drainage lines and water-sheds, are coefficient conditions on which depend the 

 sculpture of the lands. . . . The relative importance of the two conditions is especially 

 affected by climate, and the influence of this factor is so great that it may claim rank as a 

 third condition of sculpture" (116, 117). In this way Gilbert made approach to certain matters 

 of detail, the explanation of which not only exemplified the unexpected interaction of several 

 additional factors, but also illustrated the importance which comparatively small topographic 

 features had for this master of observation and analysis. No wonder that his discussion, 

 clear, logical, judicious, timely, "soon became the great classic among students of geomor- 

 phology." 



