256 ISLA. MINERALS. 



were given, tending materially to illustrate this view. 1 * 

 Every phenomenon of this nature points to some former 

 condition in the state of these rocks, under which they 

 were at times so hard as not to be displaced without 

 fracture, at others so soft and yielding as to admit, 

 not only of simple flexures, but of very complicated 

 contortion and elongation. It is not difficult to imagine, 

 that in this latter state, an extension of the same me- 

 chanical force which produced simple elongation, might 

 under peculiar circumstances draw out the softened mass 

 into the ramifying disposition which has led to the notion 

 that such rocks formed genuine veins. It is plain that 

 they afford no arguments against the posteriority of real 

 veins, whether of trap or of granite, to the stratified 

 substances which these traverse ; since the cases bear 

 no resemblance in reality, whatever they may do in 

 external and superficial aspect. 



THE only independent earthy mineral which I observed 

 in Isla was chlorite, and, as is most commonly the case, 

 it is imbedded in quartz: the metallic minerals are of 

 greater variety and importance. 



The most conspicuous of these is lead, which has 

 been wrought at no very distant period, as well as in 

 ancient times, but is now abandoned. In consequence 

 of the absence of the last lessees of this mine, and the 

 death of the manager, no information respecting the 

 workings, or the nature of the veins in which the ore 

 was found, could be procured. Nothing can be discovered 

 from the present state of the shafts and levels, or from 

 the casual pieces of ore or veinstones which are found 

 scattered about the abandoned works. It can only 

 be ascertained, that the ore consists of galena intermixed 

 with copper pyrites, and that it lies in the limestone 



* Geol. Trans. Vol. IV. 



