SE1L. GEOLOOr. 133 



masses of trap, as well as in the veins of that substance, 

 has been noticed in various parts of this work, there is 

 in one respect an important distinction between these two 

 cases. In the veins, as in the instance under review, it 

 appears that the intrusion of the fluid mass has merely 

 produced the effect of mechanically displacing, and conse- 

 quently of entangling, the fragments of the adjoining 

 strata. Such fragments are often little changed in aspect, 

 while in other cases they undergo material alterations ; 

 these variations apparently depending on differences in 

 the condition of the intruding material which are not dif- 

 ficult to conceive. But where the fragments of schist 

 are found in the larger masses of this rock, they present 

 a very different appearance ; undergoing those gradual 

 changes which mark the commencement of fusion; and 

 even the same fragment, in some instances, offering an 

 example of each substance; the trap and the schist being 

 connected by an uninterrupted gradation. It is easy to 

 understand how the latter effect should have resulted 

 from the greater bulk of the fused material, or from a 

 termination of the process of fusion in any given mass 

 of schist, before the whole change was completed. 



From the circumstances detailed in the description of 

 Sky, it has also appeared that the shales of the secondary 

 strata have been in many cases subjected to this pro- 

 cess ; thus at times producing stratified rocks resembling 

 basalt, and, at others, being in all probability completely 

 converted into amorphous masses of this or of the analo- 

 gous substances. From this latter event may be explained 

 a circumstance peculiar to the trap rocks, and although 

 but occasionally found, strongly distinguishing them from 

 all the other rocks which do not contain organic sub- 

 stances. This is the existence of carbon, or of charcoal 

 under some form, in their composition ; a fact observed 

 by chemists. It is easy t;> imagine that in this case the 

 trap has probably originated in bituminous shale, or in 

 other secondary strata containing carbonaceous and organic 



