1889.] 
383 
[Davis and Wood. 
ticularly resistant rock ; the softer rocks next up stream being widely- 
opened while only canons are eroded on the harder ones. We 
have in New Jersey similar local and temporary baselevels, whose 
general recognition will greatly improve the popular conception 
of our topographic features. The hard sandstones of Kittatinny 
mountain form such a baselevel for the up-stream country % of the 
Delaware river ; and the trap sheets of the Watcliung mountains 
exert similar control over the upper basin of the Passaic. Powell 
on another page speaking of the Appalachians, says “the base 
level of erosion of the entire area would have been the level of the 
sea :” x this is the general and ultimate sense of the term ; to this 
final baselevel must all temporary and local baselevels be re- 
duced, if time be allowed. 
Other American geologists have also used the term in two senses. 
Dutton uses it in the plural when he refers to the base levels of a 
river system, thus indicating the recognition of local controls in 
different parts ; 1 2 and he also says : “all regions are tending to base- 
levels of erosion, and if the time be long enough each region will, 
in its turn, approach nearer and nearer, and at last sensibly reach 
it. The approach, however, consists in an infinite series of approx- 
imations like the approach of an hyperbola to tangency with its 
asymptote .” 3 
Several European writers have suggested terms closely allied to 
the one under discussion. La Noe and Margerie 4 use “niveau de 
base” in the same general and local senses as attach with us to 
baselevel ; but it seems to me that they have misapprehended 
Powell’s definition of the term, for they quote from him only that 
part of his statement that refers to the temporary baselevel. Heim 
at an earlier date wrote of the mouth of a river basin as the “ba- 
sis” for valley making in the whole area concerned . 5 But the 
conception of the relation of denudation to its fundamental con- 
trolling datum plane does not yet appear to be general or definite 
enough to call for the introduction of a special term, such as base- 
level, with which to name it in text books of geology. It is still 
customary to illustrate the work of erosion with examples of its 
1 1 . c., 209. 2 High Plateaus of Utah, 1880, 28, 45. 
8 Tertiary history of the Grand Canon District. U. S. G. S., Monogr. II, 1882, 76. 
4 Les Formes du Terrain, Paris, 1888, 54, 57. 
8 Mechanismus der Gebirgsbildung, 1878, 1 , 296. 
