CHANGES PEODUCED BY AIR AND ^VATER. 37 



raised out of the ocean, must have a simple, regular sea- 

 margin ; but a surface varied by table- lands and intersected 

 br mountain-chains must, when raised out of the ocean, 

 have an outline extremely irregular both in its leading 

 features and in its details. Thus endless is the accumula- 

 tion of geological and geographical results slowly brought 

 about by this one cause — the contraction of the Earth. 



When we pass from the agency w^hich geologists term 

 Igneous, to aqueous and atmospheric agencies, we see the 

 like ever-growing complications of effects. The denuding 

 actions of air and water have, from the beginning, been 

 modifying every exposed surface ; everywhere causing 

 many different changes. Oxidation, heat, wind, frost, 

 rain, glaciers, rivers, tides, waves, have been unceasingly 

 producing disintegration ; varying in kind and amount ac- 

 cording to local circumstances. Acting upon a tract of 

 granite, they here work scarcely an appreciable effect ; 

 there cause exfoliations of the surface, and a resulting heap 

 of debris and boulders ; and elsewhere, after decomposing 

 the feldspar into a white clay, carry away this and the ac- 

 companying quartz and mica, and deposite them in separate 

 beds, fluviatiie and marine. When the exposed land con- 

 sists of several unlike formations, sedimentary and igneous, 

 the denudation produces changes proportionably more he- 

 terogeneous. The formations being disintegrable in different 

 degrees, there follows an increased irregularity of surface. 

 The areas drained by different rivers being differently con- 

 stituted, these rivers carry down to the sea different com- 

 binations of ingredients; and so sundry new strata of 

 distinct composition are formed. 



And here indeed we may see very simply illustrated, 

 the truth, w^hich we shall presently have to trace out in 

 more involved cases, that in proportion to the heterogeneity 

 of the object or objects on which any force expends itself, 

 is the heterogeneity of the results. A continent of com 



