OPHITES AND RELATED ROCKS. 25 



3rd. It has been buried at great depths, and presumably in 

 a somewhat softened condition thereby rendering it, especially 

 where fibrously divided or intersected by cracks (as it frequently 

 is), an easy prey to dissolving and decomposing agents. 



Moreover it is generally admitted by chemical geologists 

 that thermal waters containing carbonic acid, or a carbonate in 

 solution, are present in deep-seated rocks. It must also be 

 considered that the terms soluble and insoluble are merely con- 

 ventional, being applied to substances only known as such in 

 the laboratory, and that they do not preclude the idea that 

 substances capable of resisting powerful acids at the earth's 

 surface maybe unable to resist the weakest when existing under 

 the conditions stated. 



Notwithstanding the difficultly soluble character of silicate of 

 magnesia, it has been " proved " by Bischof that an artificial 

 preparation of the kind c ' when dissolved in water by carbonic 

 acid " is decomposed : he thinks, however, that this ' f is owing 

 to the silica being in the soluble modification." While " the 

 natural silicate of magnesia (steatite, the silica of which is in 

 the insoluble modification) , when finely powdered and suspended 

 in water containing carbonic acid for twenty-four hours, did 

 not show the slightest effervescence"*". 



But even absolute decomposition of serpentine was proved some 

 thirty years ago by the Professors Rogers, who submitted several 

 mineral silicates (the present one being of the number) to the 

 analytic action of carbonated, and even simple water : the result 

 in every case was a residue of magnesian and other carbonates, 

 showing that decomposition had taken place f. 



There is one mineral, peridote, which is frequently converted 

 into serpentine. If it were a pure silicate of magnesia, all that 

 would be required to effect such change would be the hydration 

 of the latter compound ; but as it contains a considerable quan- 

 tity of protoxide of iron, this may have been peroxidized, or 

 changed into a carbonate, in the one case by the addition 

 of oxygen, in the other by carbonic acid contained in the 

 penetrating water, a portion of the magnesia being removed at 

 the same time. The process is so simple as to excite surprise 

 that it has been unnoticed by the advocates of the envelopment 

 doctrine. 



* Chem. and Phys. Geology, vol. i. p. 3, vol. iii. p. 164. 



t Edinburgh New Philosophical Journal, vol. xlv. pp. 163-108 (1848), 



