Davis and Wood.] 
382 
[Nov. 20, 
maturity of the region, and they may not have been quite smoothed 
down even at their latest stage ; but the weaker rocks, such as the 
limestones of the Highlands and the red shales of the Triassic belt 
were probably worn down very flat, for they are so easily weathered 
that they must have been reduced close to a baselevel surface at a 
relatively early date ; just as at present they have been cut deep 
in valleys and widely opened in lowlands before the gneisses and 
traps have lost all indications of the extinct plain from which the 
valleys and lowlands have been eroded. 
16. General conception of baselevels and baselevel plains. The 
definition of baselevel may be purely geometrical, and the ulti- 
mate form of the surface developed on it might theoretically be a 
flat plain ; but such conceptions are too rigid for application in 
geology and geography. The Schooley baselevel that we have 
been considering must not be regarded as a definite surface, abso- 
lutely fixed with reference to the Highland mass ; it was more 
likely the average position of many small oscillations of sea-level 
down towards which the ancient land mass was gradually reduced ; 
and the Schooley baselevel peneplain was rather a general ap- 
proach to a smooth surface, prevailingly low and nearly featureless, 
but by no means a surface of geometrical uniformity, absolutely 
coincident with its controlling baselevel. While it might be possi- 
ble that endless time and stationary attitude would result in geo- 
metrical simplicity of land form, we need not expect to find such 
a result in natural occurrence, for the land is too unsteady to allow 
it. 
When Powell first introduced the term “base level,” he em- 
ployed it in two senses. He said “ we may consider the level of 
the sea to be a grand base level, below which the dry lands cannot 
be eroded this is the general and persistent baselevel of a region 
so long as the relative level of the land and sea does not change. 
He then adds, “but we may also have, for local and temporary 
purposes other base levels of erosion, which are the levels of the 
beds of the principal streams which carry away the products of ero- 
sion.” 1 The context shows clearly that these two uses of the term 
are employed. These “local baselevels of erosion” are described, 
in ascending the Colorado and Green rivers, 2 each one being de- 
termined by the passage of the stream for a distance over par- 
1 Colorado River of the West, 1875, 203. 
2 1. c., 207. 
