196 ; REVIEWS 
the peneplain are a few remnants above the general level. The Cum- 
berland base-leveling epoch came to an end with the uprising at the 
end of the Cretaceous. 
The Highland Rim is the peneplain next below the Cumberland. 
It retains a very uniform height, the difference between the northern 
and southern edges being but little more than existed during the period 
in which it was base leveled. Upon this plateau also there are monad- 
nocks which represent areas of more resistant rocks. 
The altitude of the lowest and youngest peneplain is 700 feet at the 
south and 800 feet at the northern edge. Here, as upon the other 
plateaus, there are considerable variations in altitude in different parts 
of the peneplain. ‘These should not be taken as indicating distinct 
base levels, but simply the influence of local conditions. . 
Hayes considers two hypotheses in explanation of these peneplains, 
namely, subaérial denudation and marine denudation. He finds sup- 
port for the former only. 
The streams of this region belong to three distinct river systems, 
the Cumberland, the Tennessee, and the Coosa. They are the main 
agents which have shaped the present topography. There have been 
periods of stability and relative inactivity, alternating with great revo- 
lutions. It is hard to follow all these changes in detail, for the history 
of each change is in some measure obscured by that of the next. The 
first cycle of erosion resulted in the formation of the Cumberland 
peneplain. This cycle began when the land was raised at the end of 
the Carboniferous, and ended with the uplift closing the Cretaceous. 
’ This long period of erosion was not a single cycle, but was composed 
of a number of more or less distinct cycles, the evidence of which 
remains even to this day. Hayes has worked out the general courses 
of the Paleozoic streams in some detail, but no statement would be 
intelligible without the maps. 
When the Cumberland peneplain was raised and warped, and the 
second cycle of erosion inaugurated, there were signs of activity all 
along the line. The sluggish streams began again to cut their beds 
and to fight for the mastery of favorable positions. The development 
of new streams at the expense of the old, changes in the direction of 
drainage, and final, almost perfect, adjustment of the streams in this 
cycle are carefully worked out by the author. This second cycle, while 
much shorter than the first, extends over a vast period of time. It 
ended, as did the Cumberland, by a rise of the land and a slight 
