96 
ELEMENTS OF GEOLOGY. 
CHAPTER VI. 
DENUDATION. 
Denudation defined.—Its Amount more than equal to the entire Mass of strat¬ 
ified Deposits in the Earth’s Crust.—Subaerial Denudation.—Action of 
the Wind.—Action of Running Water.—Alluvium defined.—Different 
Ages of Alluvium.—Denuding Power of Rivers affected by Rise or Fall of 
Land. —Littoral Denudation. —Inland Sea-cliffs. —Escarpments. —Subma¬ 
rine Denudation.—Dogger-bank.—Newfoundland Bank.—Denuding Pow¬ 
er of the Ocean during Emergence of Land. 
Denudation, which has been occasionally spoken of in the 
preceding chapters, is the removal of solid matter by water 
in motion, whether of rivers or of the waves and currents of 
the sea, and the consequent laying bare of some inferior rock. 
This operation has exerted an influence on the structure of 
the earth’s crust as universal and important as sedimentary 
deposition itself; for denudation is the necessary antecedent 
of the production of all new strata of mechanical origin. 
The formation of every new deposit by the transport of sed¬ 
iment and pebbles necessarily implies that there has been, 
somewhere else, a grinding down of rock into rounded frag¬ 
ments, sand, or mud, equal in quantity to the new strata. 
All deposition, therefore, except.in the case of a shower of 
volcanic ashes, and the outflow of lava, and the growth of 
certain organic formations, is the sign of superficial waste 
going on contemporaneously, and to an equal amount, else- 
w^here. The gain at one point is no more than sufficient to 
balance the loss at some other. Here a lake has grown shal¬ 
lower, there a ravine has been deepened. Here the depth 
of the sea has been augmented by the removal of a sand¬ 
bank during a storm, there its bottom has been raised and 
shallowed by the accumulation in its bed of the same sand 
transported from the bank. 
When we see a stone building, we know that somewhere, 
far or near, a quarry has been opened. The courses of stone 
in the building may be compared to successive strata, the 
quarry to a ravine or valley which has suffered denudation. 
As the strata, like the courses of hewn stone, have been laid 
one upon another gradually, so the excavation both of the 
valley and quarry have been gradual. To pursue the com¬ 
parison still farther, the superficial heaps of mud, sand, and 
gravel, usually called alluvium, may be likened to the rub- 
