356 George P. Merrill — On Roch-iceathering . 



rock, passes within the space of 2 cm. from a fine red-brown 

 unctious clay to a massive fresh unchanojed feldspar : the nepheline 

 of the nepheline syenite of Mias in the Urals is eaten out below the 

 surface of the more refractory feldspar, mica, zircons, etc., but gives 

 in the closed tube only traces of water and shows under the micro- 

 scope only incipient stages of hydi'ation. 



My own view of the case is, that the two phenomena are due to 

 widely different causes ; that serpentinization is a deep - seated 

 process due to waters or vapours coming from considerable depths, 

 and it may be even constituents of the magmas at the time of their 

 intrusion. The almost complete absence of oxidization products 

 in fresh serpentine is indicative of this. Again, serpentinization 

 is a process involving a greater degree of hydration than is 

 weathering. The serpentinized olivine rock of the Nijni Tagilsk 

 platinum district yields 14*21 per cent, of water, while the brown 

 crust which results from its weathering yields but 11-74: per 

 cent. The serpentine of Harford County, Maryland, further, 

 shows a loss on ignition of 18"15 per cent.,^ wliile the hard 

 red-brown crust produced on the immediate surface, through 

 weathering, loses but 11-82 per cent, and the residual soil but 

 7-89 per cent. In the same manner soils derived by weathering 

 from other highly hydrated magnesian rocks, as the so-called soap- 

 stone (altered pyroxenites), show a very considerable loss of water, 

 even though the actual loss on ignition in the resultant soil may be 

 a trifle greater than in the fresh rock. 



These facts are mentioned in detail here since they do not seem to 

 have been before noted, or at least their significance not realized. 

 If serpentinization were a product of weathering, a shorter exposure 

 to atmospheric influences, as in India, ought to result in merely 

 a thinner coating of serpentine over the altered material. Were it 

 a result of subaerial agencies, certainly the superficial alteration of 

 olivine into serpentine ought to be more nearly universal. Yet, in 

 several instances, as notably in the corundum areas of western 

 North Carolina, the olivine is almost perfectly fresh, but on the 

 immediate surface rotted away to a ferruginous clay. 



That this process of hydration, hydrometamorphism, or alteration, 

 whichever term may be used, is quite distinct from true weathering, 

 is further shown by the corundum crystals of this same region. 

 These are often superficially or quite altered to damourite. Yet 

 both the corundum and the alteration products are so resistant to 

 weathering, that thoy remain among the residues in the clays 

 resulting from the decomposition (weathering) of the mother rock, 

 whatever that may have been. Serpentine pseudomorphs after 

 pyroxene are sometimes more refractory than the rock in which they 

 are formed, and may be found. with well-preserved crystal outlines 

 in the debris resulting from its breaking down.- When rocks like 



^ In part due to C 0, though the amount of this constituent cannot be over 3 or 

 4 per cent. 



- J. Smith, " Crystals from Decomposed Trap " ; Geol. Mag., Dec. IV, Vol. VI 

 (1899), p. 93. 



