32 Denudation. 



I merely give this instance to show what I mean by 

 denudation produced by running water. At one time 

 the channel did not exist. The river has cut it out, 

 and in doing so, strata some of them formerly one 

 hundred and sixty feet beneath the surface have been 

 exposed by denudation. Possible, but very uncertain 

 calculations, show that to form this gorge a period at 

 the least of something like thirty-five thousand years 

 has been required. This is an important instance, and 

 it is similar to many other cases constantly before our 

 eyes, on a smaller scale, which rarely strike the ordi- 

 nary observer. 



Eefer to fig. 6, and suppose that we have differ- 

 ent strata, 1, 2, 3, and 4, lying horizontally one above 

 the other, together forming a mass several hundreds 

 of feet in thickness. The running water of a brook 



FIG. 6. 



4 

 3 



2 a 

 1 _ 



or river by degrees wore away the rocks more in 

 one place than another, so that the strata 3, 2, and 1, 

 were successively cut into and exposed at the surface, 

 and a valley in time is formed. This is the result 

 of denudation. 



Or to take a much larger instance. The strata that 

 form the outer part of the crust of the Earth have, in 

 many places, by the contraction of that crust due to 

 cooling of the mass, been thrown into anticlinal and 

 synclinal curves. A synclinal curve means that the 

 curved strata are bent downwards as in 1, Fig. 7, an 

 anticlinal curve that they bend upwards as in 2. 

 The whole were originally deposited horizontally, con- 



