132 OVERLYING AND TRAP ROCKS. 



Turning now to the chemical effects produced on 

 the adjoining rocks by these veins, there are few 

 changes in those where the overlying rocks appear to 

 be of the most distant date ; as are the porphyries 

 connected with granite and the more antient primary 

 strata. To remind the reader of the cause, is to con- 

 firm the general theory. The strata traversed or in- 

 vaded by the more antient porphyries and traps, are 

 precisely those which owe their condition to heat, and 

 which have therefore already undergone nearly all the 

 effects of which they were susceptible. Yet, even in 

 these cases, I have observed the induration of the 

 strata in contact, and the occasional entanglement of 

 portions of the surrounding rock; as in a porphyry 

 vein in Glen Fernat, thus including fragments of 

 quartz. In the more recent traps, these several cir- 

 cumstances are common ; and they present indeed 

 some of the most remarkable appearances which cha- 

 racterize and distinguish these substances. Thus, 

 ordinary sandstone becomes indurated into compact 

 quartz, calcareous and argillaceous sandstones are 

 converted into one species of chert, and argillaceous 

 limestones into another, pure limestones become 

 crystalline and large grained, with the disappearance 

 of organic remains, while the shales are converted 

 into siliceous schist. Some of the ordinary clays be- 

 come red from the ultimate oxydation of their iron, 

 while others are converted into that jasper so often 

 confounded with pitchstone. Thus is pyrites occa- 

 sionally decomposed, and coal either charred or con- 

 verted into plumbago : while these changes occur 

 equally, if to different extents, whether the contact be 

 that of masses or veins, and while, also, they are all 

 such as would be expected from the action of heat ; 

 coinciding therefore with the various phenomena 



