George P. Merrill — On Rock-weathering. 355 



This is essentially the ground taken by Lindgren also,^ and while 

 it may be open to criticism, nothing better, so far as the writer is 

 aware, has been suggested. 



Let us take for purposes of discussion the serpentinization of the 

 niinei'al olivine, as mentioned in the abstract of Mr. Holland's paper. 

 The author states (p. 31) : — "In all these cases, however, although 

 the action of the atmosphere is so striking, the results are purely 

 superficial, and a specimen of rock taken from within a few inches of 

 the clay products seldom shows a trace of hydrous decomposition, 

 even in thin sections under the microscope. This is just as true for 

 such delicate minerals as olivine and nepheline as for the commoner 

 silicates. In many of the basic dykes, certainly pre-Cretaceous and 

 probably Lower Palaeozoic in age, the absence of serpentine is so 

 complete that unusual precautions are often necessaiy for the 

 determination of the olivine, whilst in the numerous occurrences of 

 dunite throughout the Madras Presidency serpentine is extremely 

 scarce." 



Mr. Holland would account for this wide difference in the 

 conduct of the olivine in Indian and European localities on the 

 supposition that the European areas had, during the later geological 

 periods, been submerged below the sea, while in southern India there 

 are no evidences of any such depression since Lower Palaeozoic time. 



Just what would be the effect of prolonged submergence in sea- 

 water on a mass of olivine rock the writer is not prepared to say, 

 but he does express the doubt if simple weathering of the mineral 

 is ever productive of serpentine. Has there been ever advanced 

 any proof that serpentinization is a superficial phenomenon ? If 

 due to weathering we ought somewhere to meet with masses which 

 are superficially converted into serpentine and gradually resume 

 their normal character at greater depths. 



So far as is to be judged from available literature the olivine 

 granules in any mass of rock, however dense, show no gi'eater 

 degree of hydration (serpentinization) near the surface or along 

 contacts where surface waters would most readily permeate, than in 

 the interior of the mass. Further than this, serpentinization is as 

 common, so far as the United States is concerned, in the arid 

 portions of the West as in the humid East. In fact, this phenomenon 

 is apparently entirely independent of climatic conditions. Is not 

 the inference, then, fair that the process has gone on entirely 

 independent of surface waters ? 



Although convinced that excepting in the purely physical 

 weathering of arid regions hydration is a most important factor,^ 

 the absence of any but a very narrow zone of hydration products 

 between the residual clay and fresh rock is not at all strange. 

 Much depends on the texture of the rock and its mineral com- 

 position. In cases where the weathering is largely in the nature of 

 solution and oxidization such abrupt changes are to be expected. 

 The Fourche Mt. syenite, of Arkansas, an almost purely feldspathic 



1 17th Ann. Rep. U.S.G.S., 1895-6, pt. ii, pp. 90-6. 



2 " Eocks and Eock- weathering," pp. 188, 234, etc. 



