66 DENUDATION OF ROCKS. [Oh. VI 



CHAPTER VI. 



DENUDATION. 



Denudation defined — Its amount equal to the entire mass of stratified deposits in 

 the earth's crust — Horizontal sandstone denuded in Ross-shire — Levelled sur- 

 face of countries in which great faults occur — Coalbrook Dale — Denuding power 

 of the ocean during the emergence of land — Origin of Valleys — Obliteration of 

 sea-cliffs — Inland sea-cliffs and terraces in the Morea and Sicily — Limestone 

 pillars at St. Mihiel, in France — In Canada — In the Bermudas. 



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. Geologists have perhaps been seldom in the 

 habit of reflecting that 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 inseparable accompaniment of 

 the production of all new strata of mechanical origin. The formation 

 of every new deposit by the transport of sediment and pebbles necessa- 

 rily implies that there has been, somewhere else, a grinding down of rock 

 into rounded fragments, sand, or mud, equal in quantity to the new 

 strata. All deposition, therefore, except in the case of a shower of vol- 

 canic ashes, is the sign of superficial waste going on contemporaneously, 

 and to an equal amount elsewhere. ; The gain at one point is no more 

 than sufficient to balance the loss at some other. Here a lake has grown 

 shallower, there a ravine has been deepened. The bed of the sea has in 

 one region been raised by the accumulation of new matter, in another 

 its depth has been augmented by the abstraction of an equal quantity. 



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 comparison still 

 farther, the superficial heaps of mud, sand, and gravel, usually called 

 alluvium, may be likened to the rubbish of a quarry which has been re- 

 jected as useless by the workmen, or has fallen upon the road between 

 the quarry and the building, so as to lie scattered at random over the 

 ground. 



