ORIGIN OF MINERALS CHARACTERISTIC OF OPHITES ETC. 65 



been formed in the wet way"*. Bischof, we are strongly 

 inclined to think, would have felt no hesitation in taking this 

 view had he been acquainted with peridotiferous ophites that 

 were sediments originally. 



Supported by numerous evidences which have come to light 

 of late years through the study of ophites, we must declare 

 ourselves on the side of the opinion that the presence of peridote 

 in all such rocks is as much the result of hydrothermal and che- 

 mical changes as is the serpentine with which it maybe associated. 

 Besides, it is well known that lava contains water and steam for 

 years after its eruption : jets of vapour are still given off from 

 the lava of Jurolla, now nearly ninety years after its ejection. 



The occurrence of peridote in granites, though countenancing 

 the idea of this mineral having been formed by igneous action 

 and in the dry way, cannot be accepted as proving any thing of 

 the kind ; for it is now well known that these rocks occasionally 

 contain zeolites, serpentine^ chlorite, and other hydrous minerals, 

 which must have been formed in the wet way. Hence hydro- 

 thermal conditions, required by our theory for the production of 

 secondary minerals, have not always been absent in granites. 



Not long since we pointed out instances of peridote in the 

 amorphous or colloidal form, where it is interreticulated with 

 serpentine. It may also be mentioned that the former mineral 

 occurs in the crystalloid and crystalline conditions, as at Snarum. 

 Beautiful crystals have been found in the lavas of Vesuvius, in 

 forms proper to itself, and not pseudomorphic after those 

 belonging to another mineral : this may be regarded as proving 

 that peridote is not of secondary origin. Any argument of the 

 kind, however, is invalidated by the fact that other minerals 

 are in the same predicament. Epidote, calcite, and talc occur 

 crystalline, each as a secondary product and in forms proper to 

 itself, also as pseudomorphic after garnet &c. 



Again, the occurrence of crystals and crystalloids of peridote 

 in true igneous dykes and beds of lava is held by some to prove 

 that this mineral and the ejections containing it are of contem- 

 poraneous origin, But a formidable difficulty meets this view 

 at the outset. As generally considered, peridote is one of the 

 most refractory bodies known ; and in agreement with this fact 

 is the well- attested statement that peridote frequently occurs 

 under circumstances showing that the heat of its matrix, when 



* Chemical and Physical Geology, vol. ii. p. 358. 



F 



