ERUPTIVE ROCKS. 311 



It seeins to me tliat there is a marked distinction between the meta- 

 morphism that is found in regions where igneous rocks abound (and which 

 is generally admitted to be the result of the combined action of heat, press- 

 ure, and water) and that which involves the entire absorption and assimila- 

 tion of foreign rock masses into the substance of the igneous mass itself 

 The former in its extreme phase supposes a simple rearrangement of the 

 materials of a rock, a change in their form without any essential change in 

 their chemical composition, and involves at most the bringing of them to a 

 viscous state, not to that of fusion. The latter must be a dry process and 

 involves a fusion of the foreign materials as complete as thai of the original 

 magma in the deep-seated source from which it came. For fusing the 500 

 cubic miles of sedimentary beds supposed to have been assimilated by the 

 Cottonwood granite body an enormous amount of heat nuist have been 

 abstracted from that body. Now, to have this amount of heat to yield up, 

 and yet to be able to maintain itself in a state of fusion long enough to 

 ci-ystallize in the same way that it would without this addition of foreign 

 material, supposes an amount of original heat stored up within its mass 

 that ought to have vitrified some of the rocks through which it passed. 

 It is not difficult to conceive of such heat in the deep-seated source from 

 which the igneous rocks came, but that it should still exist in these rocks 

 when they have reached the point where they are ready to solidify, and 

 which may be assumed to be near the limit that this heat would carry them, 

 seems highly improbable. The only cases of actual vitrification of inclosed 

 fragments in igneous rocks that I have read of have been in recent volcanic 

 rocks, where the fragments were extremely snuxll. 



As suggested above, the pressure under which the intrusive rocks of 

 the Mosquito Range were consolidated would necessitate a higher tempera- 

 ture to produce fusion. In the case of the Cottonwood granite the pressure 

 under which consolidation took place and the consequent temperature of 

 the fusion point must have been greater still. But the Mosquito porphyries 

 retained a very fluid condition, and therefore a temperature higher, as com- 

 pared with the fusion point, than the Cottonwood granite, for a very long 

 time, since they were spread out in thin sheets and ramif^^ing bodies in 

 every direction at considerable distance from the central mass, while the 



