George P. Merrill — On Rock-iceathering. 357 



the Nijai Tagilsk peridotites, which are largely serpentiaized but 

 still coataia residuary olivines, undergo actual weathering, the 

 olivines do not yield serpentine, but break up, as does the serpentine 

 itself into free iron oxides, free silica, carbonates of magnesia, and 

 unrecognizable earthy products. 



The ideas put forward in this paper are not altogether new, and 

 are in part in harmony and in part in conflict with those advanced 

 by others. 



Thus Bischoff says : ^ " The decomposition of a rock takes place 

 only when it is exposed to the undisturbed action of atmospheric 

 agents ; while alteration, on the contrary, takes place when the rock 

 is more or less beyond the reach of this action." On the other 

 hand, Eoth,' to whom we owe so much, speaks of both serpentine 

 and zeolites as products of weathering {Verwitterung) , but in his 

 latest work ^ he so far changes his views or mode of expressing them 

 as to call the deep-seated process Complicirte Verwitterung, in dis- 

 tinction from the purely superficial Verwitterung, due to atmospheric 

 action. Moreover, in the paper " Ueber den Serpentin," to which 

 reference is made above, he describes the processes and results 

 incidental to the Verwitterung of the serpentine quite independently 

 of those incidental to its first production. 



Teall * apparently accepts unhesitatingly ideas contrary to those 

 expressed by myself, for he says : " The alteration of olivine by 

 surface agencies — water, carbonic acid, and oxygen — gives rise to 

 serpentinous and other pseudomorphs." 



Considei'ation along these lines led the present writer in tbe work 

 to which reference has been made to attempt making a distinction 

 between true weathering and the more deep-seated process which he 

 called hydrometamorphism. Sui'ely processes so widely different as 

 those resulting in the production of a serpentine from a peridotite 

 and the final destruction through oxidation, carbonization, and partial 

 dehydration of this same serpentine, should not be confounded 

 under the same name. The distinction, it is true, is not one that 

 can at all times be readily made. As the petrologist finds difficulty 

 in separating his plutonic from the effusive rocks, so here are no 

 hard and fast lines, and it is often impossible to state at just what 

 point one shall assume that superficial processes cease and the deep- 

 seated begin. The writer's conclusions are to the effect that the 

 zone of oxidation forms the natural and easiest recognizable limit. 

 The processes within this zone are those of weathering; those below, 

 whether brought about by superficial waters deprived of their free 

 oxygen and carbonic acid, or by deep-seated waters welling upwards, 

 are those of hydrometamorphism, metasomatosis, alteration, or 

 whatever suitable name may be adopted. 



1 " Cliemical and Physical Geology," Paul & Drummond's English translation, 

 1854, vol. iii, ^. 86. 



2 " Uber den Serpentin, etc." : Abhandl. der K. Akademie der Wisa. zu Berlin, 

 ii (1869), p. 42. 



^ " Allegemeine u. Chemische Geologic," 1893. 

 * " British Petrography," p. 85. 



