312 ON THE IGNEOUS METAMORPHOSIS [Ch. XIV. 



science ; for, if we attempt to pursue a path of our own, re- 

 jecting the aid of the kindred sciences, vague and groundless 

 speculations will speedily usurp the place of cautious and 

 rigid inductions. 



The metamorphosis, however, which is supposed to have 

 been produced on the strata by granite, has not, according to 

 this theory, been confined to the point of contact, but has 

 extended to very considerable distances therefrom ; converting 

 sandstones into gneiss and mica-slate, shale into clay-slate, 

 and the latter into hornblende, schist, and various other 

 crystalline slates. 



In order to conceive the possibility of such an occurrence, 

 we are called upon to imagine voluminous masses of melted 

 granite to have been for ages in an incandescent state, in 

 contact with sedimentary deposits : by which the latter, to 

 the whole extent of the primary slates, were intensely heated 

 for a long period, but nowhere to the point of fusion, though 

 this is said to have been nearly accomplished in the case of 

 gneiss. 



That a volume of melted lava does remain for a long time 

 in contact with the stratified rocks in volcanic vents, can- 

 not be denied ; but the amount of change thereby produced 

 does not seem to have been ascertained : indeed, in this case, 

 it cannot be easily examined ; but, in the dykes which com- 

 municate with immense superimposed beds of trap, we might 

 expect to find some data on which to form an estimate of this 

 power, more especially as such trap is supposed to have been 

 ejected from beneath under considerable pressure ; therefore 

 intensely heated, and under circumstances favourable to the 

 metamorphosis of the adjacent strata. But these are not 

 always changed; on the contrary, these igneous and aqueous 

 rocks often lie side by side, undisturbed and unaltered. 



It appears to us to be a conclusion too hastily drawn, that 

 the granite has been in a state of fusion subsequent to the 

 supposed deposition of the primary strata : for it follows, that 

 such aqueous sediments must have been formed on a basis of 



