548 W. M. DAVIS — DATES OF TOPOGRAPHIC FORMS. 



will acquire features characteristic of the newer cycle, while the harder mem- 

 bers retain features from one or more previous cycles. This corollary is the 

 guiding principle in practical work. 



A natural consequence of the continued attack of the destructive forces of 

 geographic development is that the more ancient forms are consumed and 

 obliterated in the production of the new ones. It is therefore characteristic 

 of this kind of work that, as more and more modern time is approached, the 

 recognition of finer and finer subdivisions of time becomes possible. The 

 older cycles that I have identified give us little record now beyond the gen- 

 eral statement of a long still-stand. All the presumable minor oscillations 

 during such a period are lost to our belated sight. 



All old land forms would be rubbed out were it not for their occasional 

 preservation by burial for a long period, followed by a resurrection, when 

 they may become once more visible. Such surfaces might be referred to 

 two dates : one the date of their first development and burial, the other the 

 date of their rediscovery. 



It appears that, when thus regarded, the forms of the land around us have 

 been produced at different times. It is therefore possible to date them in 

 accordance with the geological ages or divisions of time in which they were 

 given their existing forms. In this geological sense, their " age " has an 

 entirely different meaning from the geographic sense in which it has been 

 used above. I shall therefore use the word ** date " in speaking of geological 

 time, and reserve "age" for the geographic meaning already indicated. 



The Topographic Forms of the Atlantic Slope. 



Outline of this Essay. — The thesis maintained in this essay is, in brief, as 

 follows : The Permian and Jurassic constructional topography of the Atlantic 

 slope was practically obliterated over the greater part of the area by the 

 long-continued denudation of Jurassic and Cretaceous times, as a result of 

 which the region was reduced to a lowland of faint relief — a peneplain. 

 The only considerable elevations that remained above this lowland were in 

 the White mountains of New Hampshire and the Black mountains of North 

 Carolina, with their extension in the Blue ridge of Virginia. This Creta- 

 ceous lowland was uplifted about the opening of Tertiary time, and consti- 

 tutes the upland surface of our highlands. The valleys and open lowlands 

 of to-day have been etched during Tertiary time in the uplifted Cretaceous 

 peneplain, their depth depending on the height to which their streams were 

 raised, and their width depending on the weakness of the rocks in which they 

 are sunk. About the close of Tertiary time a moderate elevation occurred, 

 allowing the rivers and streams to trench the lowlands produced in the Ter- 

 tiary cycle. Near the sea-coast, where changes of level soon have effects of 



