96 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
occupied the larger valleys between the doabs, has been carried 
away by the streams. Some of it may be seen in small deltas at 
the river mouths, the rest is spread over the sea door. In the 
future, the doabs will be more dissected by ravines and valleys; in 
the past, they were less dissected. Before any valleys were cut, the 
doabs were all united in a continuous coastal plain. When all this 
has been made clear, no one can doubt that the coastal plain was 
once a smooth sea bottom, and that this region was then like the 
sea-skirted mountains of the first model. Since then, the relative 
level of land and sea has been altered, part of the even sea bottom 
is now revealed to form the coastal plain, and the valleys have been 
cut in the plain by the streams. 
As soon as this simple relation is recognized, the background of 
hills and mountains is seen to be the oldland , from whose waste 
the strata of the coastal plain were built. The inner boundary of 
the plain is the oldland shore; and the former sea cliffs may be 
discovered along- the margin of the hills. The cliffs are no longer 
clean swept at the base by waves; they are now cluttered with 
waste fallen from their faces. The streams from the oldland no 
longer mouthe at the oldland shore, but extend their courses across 
the coastal plain, guided by its gentle slope; for this reason, they 
may be called extended streams. As their direction is consequent 
upon the slope of the plain, they may also be called consequent 
streams. Some small streams may rise on the plain, and flow by 
short direct courses to the sea; these are original consequent 
streams , in contrast to the consequent extensions of the streams 
that head on the oldland. The latter are the larger; some of them, 
draining a large extent of oldland, may be the master rivers of the 
district. 
As the region now stands relatively higher than before, the rivers 
all tend to cut down their valleys to the new level of the sea at their 
mouths. This controlling base of their action is called the base- 
level of the region. As long as the land stands in its present posi¬ 
tion, continually wasting under the destructive attack of the atmos¬ 
phere, the surface will be worn lower and lower, closer and closer to 
baselevel. In the coastal plain shown in the model, a good beginning 
of this great task is accomplished along the line of the streams, but 
the interstream doabs are as yet hardly touched. The general pro¬ 
cess of wasting by which the land is worn down, and deeper and 
deeper structures laid bare, is called denudation. It vhries with 
