OF UNSTRATIFIED ROCKS AND VEINS. 161 



tween these two substances is easily accounted for. 

 It is unnecessary at present to support this case by 

 facts, as it will better corne under review hereafter; 

 but even where the composition is similar, experiment 

 has taught us that the same mixture will, under dif- 

 ferent degrees of heat, and with a different management 

 of it, produce different artificial rocks. Slow cooling 

 generates a highly crystalline arrangement ; quick, an 

 obscure one. The reader may be safely allowed to 

 make use of this analogy in his own way; as the 

 writer himself could do little more than apply it hy- 

 pothetically to the explanation of the differences be- 

 tween trap and granite. Only let it be recollected, 

 that there are many differences in the conditions of the 

 earliest and latest erupted materials, and in that of 

 the strata among which they have intruded, which 

 will, with a little reflection, go far towards explaining 

 the differences in question, if the causes and effects 

 cannot be rigidly approximated. 



I will not now examine the reasons why the fluidity 

 of granite and of trap must have been a fluidity 

 of fusion ; although, as a question common to all the 

 unstratified rocks, it in some measure claims a place 

 here. It will be more conveniently examined when 

 the mineral nature of these rocks shall come under 

 review. 



Thus far this discussion has proceeded without in- 

 volving considerations of a merely hypothetical nature; 

 but it cannot be dismissed without noticing the 

 causes which Geological Theorists have assigned for 

 the fusion and protrusion of the unstratified rocks. 

 To the expansive powers of a heat, or fire, situated 

 deep beneath the surface, is attributed the enlargement 

 or protrusion of the fluid which it has produced. The 



VOL. i, M 



