728 W. N. Benson — Origin of Serpentine. 



hydration during the igneous epoch can subsequently 

 become changed to serpentine by the action of deep cir- 

 culating epigene waters, though it seems not improbable. 

 Very frequently the hydration and carbonation has 

 caused the development of concentric zones about the 

 ultrabasic mass ; in such cases the outermost is talc and 

 carbonates, the inner is serpentine, and the center anhy- 

 drous dunite. This recalls the development of greisen 

 about some but not all granite-masses, and in these the 

 outermost portions, which have been longest and latest 

 acted upon by the outward-passing volatile matter, show 

 the change most completely. This, however, is perhaps 

 the explanation only in those cases if any in which the 

 hydration can be referred definitely to the action of 

 waters actually proceeding from the ultrabasic magma; 

 in others, in which the water of hydration has been 

 derived from a latter magmatic differentiate, or perhaps 

 from the general underground circulation, we must con- 

 ceive of the peripheral alteration as the result of a type 

 of centripetal diffusion of solutions. 



It would be wrong in putting forward the evidence for 

 the significance of magmatic waters in the production of 

 serpentine, though it is widely accepted, to overlook the 

 fact that it is not accepted by certain geologists, who have 

 had very great experience of this rock, and have con- 

 cluded that the change of the anhydrous silicate is so 

 capriciously localized that it should be regarded rather as 

 the result of the ordinary change of a mineral very sus- 

 ceptible to the action of water. But while not ignoring 

 this objection, or denying the possibility that the change 

 may have been sometimes effected by waters other than 

 magmatic, the writer hopes that by this putting together 

 of the evidence that much of the serpentinization is per- 

 formed by water of magmatic origin some service may 

 be rendered to other students of this interesting and dif- 

 ficult problem. It may be strongly urged in conclusion, 

 upon those whose laboratory facilities permit of such 

 work, that much useful information might be obtained 

 from the experimental investigation of the action of car- 

 bonated waters and other energetic solutions upon mag- 

 nesian silicates at high pressures and temperatures, for 

 this seems essential for the final solution of the problem. 



The writer's thanks are due Professor Bonney, Dr. 

 Harker and to many other friends, teachers and students, 



