248 The American Geologist. October, i899 
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 sur- 
face rotted away to a ferruginous clay. 
That this process of hydration, hydrometamorphism, 01 
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 corimdum and the alteration 
products are so resistant to weathering that they remain 
among the residues in the clays resulting from the decomposi- 
tion (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 the Nijni 
Tagilsk peridotites, which are largely serpentinized but still 
contain residuary olivines, undergo actual weathering, the 
olivines do not yield serpentine, but break up, as does the ser- 
pentine 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 BischofT says if "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, Roth,]; 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 
*J. Smith. " Crystals from Decomposed Trap" : Geol. Mag., Dec. 
IV, Vol. VI(i899), p. 93. 
f'Chemical and Physical Geology," Paid & Drummond's English 
translation, 1854, vol. iii, p. 86. 
J"Ueher den Serpentin, etc."; Abhandl. der K. Akademie der Wiss. 
7.U Berlin, ii (1869), p. 42. 
§"Allegemeine u. Chemische Geologic," 1893. 
