56 REPORT—1862. 
of the earth to the direct action of convulsive force proceeding from the interior, 
rather than to their true source in the gentle, gradual, silent influence of the 
“‘ weather,” continued through an indefinite period of past time. 
I have heard even educated men speak of the correspondence in the chalk cliffs 
of the opposite sides of the Straits of lever: as evidence in favour of the notion that 
Fugiede fad been separated from France by the tearing open of those straits by what 
they called some “ great convulsion of nature.” There is hardly a description to be 
found in any book, of any deep and narrow valley or mountain gorge—especially 
if the precipices on each side of it show entering and re-entering angles, and rocks 
that were obviously once continuous across the gap,—but what its formation is un- 
hesitatingly attributed to this vague imaginary force, a “convulsion of nature.” Nay, 
I have even heard the existence of broad valleys over an anticlinal arch, such, for 
instance, as the valley of the Weald, attributed to the effect of the gaping of the 
rocks at the surface, consequent on the upward flexure of the beds. Mythical 
powers of disturbance are called into existence with as bold a personification as the 
Bia and Kpartos of the poet, and with even less warrant for their existence. 
It seems to me, therefore, that the time is come when geologists should study a 
little more closely this problem of the mode of production of the surface of the 
land, and determine exactly the method of the formation of those variations in its 
outline which we call mountains, hills, table-lands, cliffs, precipices, ravines, glens, 
valleys, and plains. 
Few men, perhaps, ever pause to inquire into the origin of a great plain; never- 
theless the question may well be put, and it is one which deserves an answer. 
Some plains are doubtless the result of original formation. They are level and flat, 
because the beds beneath the surface are horizontal. Even these, however, have 
very rarely a surface formed simply by the last-deposited beds. The actual surface 
is one that has been formed by the erosion and removal of more or less of the 
uppermost beds, and the production of undulations formed by the act of cutting 
down into the beds below. This erosion or denudation has even in many such 
cases gone to the length of entirely removing a much greater thickness than we 
sucky at first suspect, the present surface being one that has been laid bare by 
that remoyal. 
In all cases where the beds below the surface are not strictly horizontal, or do 
not accurately coincide as to their “lie ” with the form of the surface, it is obvious 
that the plain must be one of denudation. 
Suppose we take the great plain on which we now are, and which stretches 
from Cambridge far into Lincolnshire. The hills which rise from it towards the 
east are formed by the escarpment of the Chalk, the beds of which terminate abruptly 
at that escarpment, and allow the clays which lie beneath the Chalk to come up to 
the surface and spread beneath the plain. The hills rising to the west of the plain, 
on the other hand, are formed of the Oolites, the beds of which lie below these clays 
and rise gently from beneath the plain, and themselves terminate in an escarpment 
still further west. 
There can be no reasonable doubt that the whole thickness of the Chalk and the 
beds below it once spread many miles to the westward of their present boundaries. 
The little chalk-capped monticule of the Castle Hill, at the western end of the 
town of Cambridge, and the hills near Madingley show that the Chalk was once 
continuous that far, at all events, from the Gogmagogs; and, had still higher ground 
been left by the denudation still further west, that would in like manner have 
been capped by the bottom beds of the Chalk. 
The hal on which Ely stands is, I believe, an outlier of the Lower Greensand, 
the general mass of which crops out some miles to the eastward; and other hills 
rising from the plain will in like manner be found to have their summits capped 
by beds, apparently horizontal, but in reality dipping at a very gentle angle to the 
eastward, so as to ultimately cut the surface of the plain in that direction and then 
sink beneath it. All such outliers are clear proof that the beds formerly extended 
over the intervening spaces, and show us that the rocks now left in the ground are 
only a portion of those that were originally deposited. 
The great plain of the Fens, then, is one of denudation, its surface being one that 
