108 
ELEMENTS OF GEOLOGY. 
CHAPTER VII. 
JOINT ACTION OF DENUDATION, UPHEAVAL, AND SUBSIDENCE 
IN KEMODELLING THE EARTh’s CRUST. 
How we obtain an Insight at the Surface, of the Arrangement of Rocks at 
great Depths.—Why the Height of the successive Strata in a given Region 
is so disproportionate to their Thickness.—Computation of the average 
annual Amount of subaerial Denudation.—Antagonism of Volcanic Force 
to the Levelling Power of running Water.—How far the Transfer of Sed¬ 
iment from the Land to a neighboring Sea-bottom may affect Subterranean 
Movements.—Permanence of Continental and Oceanic Areas. 
How we obtain an Insight at the Surface of the Arrange¬ 
ment of Rocks at great Depths.— The reader has been already 
informed that, in the structure of the earth’s crust, we often 
find proofs of the direct superposition of marine to fresh¬ 
water strata, and also evidence of the alternation of deep-sea 
and shallow-water formations. In order to explain how 
such a series of rocks could be made to form our present 
continents and islands, we have not only to assume that 
there have been alternate upward and downward movements 
of great vertical extent, but that the upheaval in the areas 
which we at present inhabit has, in later geological times, 
sufficiently predominated over subsidence to cause these por¬ 
tions of the earth’s crust to be land instead of sea. The sink¬ 
ing down of a delta beneath the sea-level may cause strata 
of fluviatile or even terrestrial origin, such as peat with trees 
proper to marshes, to be covered by deposits of deep-sea ori¬ 
gin. There is also no end to the thickness of mud and sand 
which may accumulate in shallow water, provided that fresh 
sediment is brought down from the wasting land at a rate 
corresponding to that of the sinking of the bed of the sea. 
The latter, again, may sometimes sink so fast that the earthy 
matter, being intercepted in some new landward depression, 
may never reach its former resting-place, where, the water 
becoming clear may favor the growth of shells and corals, 
and calcareous rocks of organic origin may thus be superim¬ 
posed on mechanical deposits. 
The succession of strata here alluded to would be consist¬ 
ent with the occurrence of gradual downward and upward 
movements of the land and bed of the sea without any dis¬ 
turbance of the horizontality of the several formations. But 
