INTERPRETATION OF LAND FORMS 131 



and last terms were known, the intermediate terms and 

 the sequence remained to be established. A significant 

 contribution to this problem was made by Jukes (1862 ). 22 



"... I believe that the lateral valleys are those which were 

 first formed by the drainage running directly from the crests of 

 the chains, the longitudinal ones being subsequently elaborated 

 along the strike of the softer or more erodable beds exposed on 

 the flanks of those chains." 



Powell's discussion of antecedent and consequent 

 drainage (1875) and Gilbert's chapter on land sculpture 

 in the Henry Mountain report (1880) are classics, and 

 McGee's contribution 28 contains significant suggestions. 

 but the master papers are by Davis, 29 who introduces an 

 analysis of land forms based on structure and age by the 

 statement : 



"Being fully persuaded of the gradual and systematic evolu- 

 tion of topographical forms it is now desired ... to seek the 

 causes of the location of streams in their present courses ; to go 

 back if possible to the early date when central Pennsylvania was 

 first raised from the sea, and trace the development of the several 

 river systems then implanted upon it from their ancient begin- 

 ning to the present time." 



That such a task could have been undertaken a quarter 

 of a century ago and to-day considered a part of every- 

 day field work shows how completely the lost ground of a 

 half-century has been regained and how rapid the 

 advance in the knowledge of land sculpture since the 

 canyons of the Colorado Plateau were interpreted. 



Features Resulting from Glaciation. 

 The Problem Stated. 



Early in the nineteenth century when speculation 

 regarding the interior of the earth gave place in part to 

 observations of the surface of the earth, geologists were 

 confronted with perhaps the most difficult problem in the 

 history of the science. As stated by the editor of the 

 Journal in 1821 : 30 



' ' The almost universal existence of rolled pebbles, and boulders 

 of rock, not only on the margin of the oceans, seas, lakes, and 

 rivers; but their existence, often in enormous quantities, in 



