152 THE PRINCIPLES INVOLYEB IN EOCK-WEATHERING 



This transformation, as already noted, takes place through 

 processes that may be simple, or again complex. It is but 

 rarely that one, alone, prevails for any length of time, and as a 

 rule several or many go on together. Were it possible, it might 

 be well to consider briefly each of these in its turn and by itself. 

 From the fact, however, as above stated, that any one, either 

 physical or chemical, rarely goes on alone, it is thought best to 

 treat the subject as below, and describe in more or less detail 

 the action, first, of the atmosphere, second, of water, in both 

 the solid and liquid form, and third, that of plant and animal 

 life, finally considering the combined action of all these forces, 

 as manifested on the various types of rock which go to make up 

 the earth's crust. 



So striking a phenomenon as the breaking down, or degenera- 

 tion as we may call it, of a mass of firm rock, naturally did not 

 escape the observation of the earlier workers in this and allied 

 branches of science, and the older literature from the time of 

 Hutton contains numerous references to it, though the full sig- 

 nificance of atmospheric agencies in bringing about the results, 

 seems not at first to have been fully realized. 



The exciting cause of the degeneration, particularly in warm 

 latitudes, where phenomena of this nature are often more ap- 

 parent, has been a matter of some speculation, and at the out- 

 set it may be well to indicate in brief their tendencies. 



Fournet, as quoted elsewhere, writing as early as 1833, in- 

 sisted upon the efficacy of water containing carbonic acid in 



body, as where granitic rocks are resolved into sand and kaolinie material, 

 with liberation of carbonates of the alkalies and of lime, and oxides of 

 iron. It does not include those deeper-seated changes — changes taking place 

 below the zone of oxidation which result mainly in hydration and the pro- 

 duction of new mineral species, as chlorite, sericite, zeolites, etc., but during 

 which the rock mass as a whole retains its individuality and geological 

 identity. The distinction is not one that has been sharply insisted upon, 

 and indeed geologists and petrologists as a rule have been extremely care- 

 less in their use of such terms as alteration, decomposition, and weathering. 

 For reasons above stated and others given on p. 140, it seems best to limit 

 the terms weathering and decomposition to processes involving the destruc- 

 tion of the rock mass as a geological body, and to designate the purely 

 mineralogical deeper-seated changes as alteration, which may or may not 

 be due wholly to hydrometamorphism. This is the distinction also made 

 by Van Hise in his work on metamorpMsm, though expressed somewhat 

 differently, his "belt of weathering" being that portion of the zone of 

 katamorphism extending from the surface down to the level of the ground 

 water. 



