78 IMG OVI S SHOVES SS IMGVOY PIM IS, 
divergence of meaning has arisen with regard to it. It is there- 
fore proposed to trace its history, with the hope of inducing 
geologists and geographers to use it in a somewhat restricted 
sense. It will be spelled here as a single word, disregarding 
the separation into two words by some authors and the hyphen 
of others, except in the first quotation. 
Powell’s original definition of ‘‘ baselevel ” is as follows, the 
parenthesis being in his text : 
We may consider the level of the sea to be a grand base level, below which 
the dry lands cannot be eroded ; but we may also have, for local and tempo- 
rary purposes, other base levels of erosion, which are the levels of the beds 
of the principal streams which carry away the products of erosion. (I take 
some liberty in using the term level in this connection, as the action of a run- 
ning stream in wearing its channel ceases, for all practical purposes, before 
its bed has quite reached the level of the lower end of the stream. What I 
have called the base level would, in fact, be an imaginary surface, inclining 
slightly in all its parts toward the lower end of the principal stream draining 
the area through which the level is supposed to extend, or having the inclina- 
tion of its parts varied in direction as determined by tributary streams.) Where 
such a stream crosses a series of rocks in its course, some of which are hard, 
and others soft, the harder beds form a series of temporary dams, above which 
the corrasion of the channel through the softer beds is checked, and thus we 
may have a series of base levels of erosion, below which the rocks on either 
side of the river, though exceedingly friable, cannot be degraded (1875, 
203, 204). 
‘‘ Baselevel’’ as thus defined seems to include three ideas. 
First, the grand or general baselevel for subaerial erosion is the 
level of the sea; second, a baselevel is an imaginary, sloping 
surface which generalizes the faint inclination of the trunk and 
branch rivers of a region when the erosion of their channels has 
practically ceased ; third, local and temporary baselevels are 
those slow reaches in a river which are determined by ledges in 
its course further down stream. 
There is some reason for thinking that Powell’s intention 
may have been misunderstood with respect to the first and third 
of these ideas. The first, ‘“‘the level of the sea,’ may have 
referred only to the actual area of the sea, and not to an imagi- 
nary extension of the sea level or geoid surface under the lands. 
