W. N. Benson — Origin of Serpentine. 709 



crust, into regions of lesser pressure, and there consoli- 

 dated, they are now found generally serpentinized, while 

 those which consolidated in the deeper parts of the crust, 

 and are associated with crystalline schists (like the 

 olivinites of Norway), have formed under heavy pres- 

 sure, and are in general nearly anhydrous. The infer- 

 ence from this would be that where there was available 

 a supply of water to act upon the peridotite, hydration 

 would occur down to such a depth that the chemical 

 affinity of the olivine for water remained sufficient to 

 overbalance the constraint exercised by the consequent 

 expansion of the rock mass. In the deeper regions the 

 Volume Law became paramount, and hydration with 

 expansion could no longer occur. Anhydrous masses 

 would remain at shallow depths, where for some reason 

 there was no great supply of magmatic water, or where 

 the water was enabled to escape, to higher levels, before 

 it had thoroughly acted upon the ultrabasic rock. This 

 possible source of water of serpentinization we now pro- 

 ceed to discuss. 



c and d. Magmatic waters. 



While it is obvious, as Holland points out, that the 

 occurrence of anhydrous masses of peridotite raises 

 difficulties in the way of the general reference of ser- 

 pentinization to the action of magmatic waters, other 

 difficulties appear if we must explain away the evidence 

 in many localities pointing the action of such waters. 

 This evidence is discussed below, but as very diverse 

 features must be considered, a summary of the general 

 argument will first be given, and the grounds which sup- 

 port the several statements contained therein will be 

 detailed later. The following appear to be the salient 

 points : — 



i. Magmatic waters, highly charged with silica and carbonic 

 acid, etc., have been emitted in connection with certain 

 peridotitic intrusions, 

 ii. v Experiment shows that carbonated waters have a particu- 

 larly strong solvent action on serpentine, and are able to 

 attack olivine also, 

 iii. Where magnesian rocks have been converted into talc and 

 carbonates, this process occurred after they had been con- 

 verted to serpentine, except in those cases where such 

 change is demonstrably due to the action of atmospheric 

 weathering. 



Am. Jour. Sci.— Fourth Series, Vol. XLVI, No. 276.— December, 1918. 

 34 



