KEVIEW OF CRETACEOUS AND TERTIARY WORK. 579 



beneatli. The great variation iu the Avidth of the valley lowlands thai have 

 been opened on the course of our greater rivers serves to emphasize the rela- 

 tively small share of excavation actually performed by river action directly, 

 and the large share that is to be ascribed to the slow processes of decaying, 

 wasting, creeping, and wet-weather washing down hill. If one would picture 

 the process in operation, he has only to conceive of time as passing quickly, 

 and he may then in imagination see the waste of the land running down hill 

 as fast as the snow-banks melt under a warm spring rain ; or faster, up to 

 any desired rate. This mental exercise has its use in bringing clearly to 

 mind the sequence of forms through which a land-mass passes from its con- 

 structional youth to its baseleveled old age; and without it one can hardly 

 become familiarly acquainted with the development of our topography. 



The Tertiary valleys and lowlands mark an advance in the reduction of 

 the uplifted Cretaceous peneplain towards baselevel, but the advance has 

 not been nearly so complete as that of the long Jurassic-Cretaceous cycle. 

 That cycle accomplished as nearly a complete baseleveling of a large area 

 as any that I have yet found, although the post-Cretaceous baseleveling of 

 the plains of the upper Missouri iu Montana east of the mountains is of ex- 

 traordinary perfection.* Our uplands of to-day in their relation to sea-level 

 correspond to the hills that rose over the Cretaceous lowland, both being 

 residuals not reduced to baselevel ; but the present uplands are not so de- 

 crepit as were the Cretaceous residuals ; the relief of the harder parts of the 

 present uplands has indeed hardly reached maturity, hardly yet received so 

 great a variety of form as it will if the process of sculpture is carried on 

 farther. The valleys still have space to increase in number by ramifying 

 into the uplands ; the profile of the country is not yet so broken and varied 

 as it may come to be. 



The moderate advance in the development of the topography on the hard 

 rocks of the uplands gives us warrant for regarding the hills that rise to 

 higher levels than that of the uplifted peneplain as essentially still preserving 

 the form that they had at the end of the Cretaceous cycle, for the dissection 

 of the general upland has taken place only where the streams have trenched 

 it and produced steep slopes on which its waste goes on with some rapidity. 

 AVhile the Cretaceous hills are in all cases avoided by the streams of the 

 Tertiary cycle, their slopes are generally small, and their change of form in 

 the Tertiary cycle may therefore be almost neglected ; but in the White 

 mountains of New Hampshire and the Black mountains of North Carolina, 

 where the Cretaceous mountains remain, this statement is too moderate. 



Post-Tertiary Topography. — There remains to be considered the work of 



*See my report on the " Relation of the Coal of Montana to the ohier Rocks," Tenth Census of the 

 U. S., vol. XV, p. 710. 



