74 
surface whereon the Upper Cretaceous rocks were formed. Each 
of these plains coincides with an important unconformity. The 
First Plain, for example, coinciding with a break equal in extent 
to a thickness of five miles of strata, which had been removed by 
denudation long after the Ludlow period, and yet prior to the 
commencement of the Carboniferous epoch in the North of 
England. (See the diagrams below). 
It was pointed out that such plains may appear at the surface 
through a variety of causes. They may represent the final result 
of prolonged marine action ; or they may be due, as Dr. A. Geikie 
has pointed out, to the action of subaerial forces, which have 
reduced the original inequalities of the land, in course of time, to 
one uniform level, that of the sea, and thus have formed what Dr. 
Geikie aptly terms a “base level of denudation.” They may, 
again, owe their present position, in many cases, to the re-exposure 
of old plains of any age, even of such as have formerly been covered 
by a considerable thickness of newer strata, which has afterwards 
been removed by the prolonged action of natural causes. 
In such cases as the last, the degree of perfection in which any 
such re-exposed plain may be left is related (1) to the nature and the 
extent of the disturbances that particular plain may have undergone 
in company with its immediate overburden of new rock; (2) to the 
ratio between the durability of the rocks forming the old plain and 
that of the strata—especially of their basement beds—to whose 
removal the re-exposure of the plain is due. Where the strata 
above the plain waste no faster than the rocks below, the regularity 
of the old floor will be destroyed as fast as it is reached by the 
denudants. But where, on the other hand, the beds above the 
plain waste at a higher rate than any part of the platform they 
repose upon, the newer strata recede laterally, and leave their 
original floor in a state of perfection dependent solely upon the 
total amount of disturbance that had affected the newer rocks since 
their formation. In the case of beds that have been upheaved 
with little or no resulting disturbance, a plateau may be brought 
to light with almost as even a contour as when it was formed 
originally. If the rock, again, happen to have been tilted, then 
