66 ROCK-METAMORPHISM. 



in a molten condition, was insufficient to fuse it. The opinion 

 is therefore pretty general that the peridote in these cases has 

 been thrown out of volcanoes in a solid condition (crystalline 

 and amorphous), imbedded in the lava; in other terms, as ex- 

 pressed by Bischof, " all these facts are indications of its exist- 

 ence in a solid state in the melted lava, and are quite inconsis- 

 tent with the opinion that it has originated from the lava/'' 

 Moreover " the occurrence of peridote in a metamorphic rock 

 renders it intelligible that lava penetrating through beds in such 

 rocks might carry up with it lumps or crystals of this mineral " *. 

 But we cannot admit that its presence in lava is to be explained 

 in this way. The fact that it occurs abundantly in the old lavas 

 of Vesuvius and rarely in those of historical eruptions may be 

 held as supporting a contrary interpretation, in cases of this 

 kind at least; for, admitting that this mineral has been derived 

 from previously existing rock-masses lying at great depths, 

 there is no reason against the subterranean sources yielding it 

 at the present day. 



Before closing this portion of our present argument we shall 

 cite a passage from Bischof giving his view as to the origin of 

 other minerals (augite, hornblende, leucite, &c.) common in 

 dolerites and lavas, though it may not be unnecessary to remind 

 the reader beforehand that, in agreement with passages already 

 cited, our author must have inconsistently excluded peridote. 

 "It is evident that, after the solidification and cooling of lava, 

 crystals can no longer be formed* by fusion. Therefore, when 

 we find that the older lava contains crystals which either do not 

 occur at all in the more recent lava, or which are at least much 

 larger and better developed than in the latter, it is certain that 

 these crystals have been formed in some other way than by fusion, 

 and there is no other way than the wet way in which they can 

 have been formed : . . . . there is no reason why this mode of for- 

 mation should be considered impossible in the older lava, which 

 has for long periods been exposed to the action of water. " .... 

 "We must therefore refrain from regarding crystalline minerals 

 which occur in volcanic masses as products of fusion " f. 



In view of the various points thus far brought forward, it seems 

 most unreasonable to exclude peridote from the list of products of 

 alteration. Therefore, until better evidences than have hitherto 



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

 t Ibid. vol. ii. pp. 94, 95. 



