RUM. GEOLOGY. 491 



rock, which, like many others, possesses neither an inva- 

 riable aspect, nor a steady chemical composition, and is 

 moreover in a perpetual state of transition into other ana- 

 logous substances. To refer to any authenticated speci- 

 men is in this case no remedy, since the access to that 

 must always be limited ; besides which, an appropriation 

 so rigid would exclude the far greater proportion of speci- 

 mens, and generate a necessity for a number of additional 

 terms. In the instance just described I have been obliged 

 to apply this term to the fine grained dark rock, which 

 is indeed undistinguishable by any mode of definition. 

 Yet there is reason to suppose, from its situation among 

 the decided augit rocks, and from the crystals of that 

 substance which it contains, that like those it is com- 

 pounded of augit and felspar. It is possible that some 

 ready method may hereafter be discovered of distinguish- 

 ing this compound from one of hornblende and felspar, 

 which seems in the opinion of mineralogists to constitute 

 the genuine basalt; but at present there is no criterion 

 by which it can be certainly recognised. It must there- 

 fore be suffered to remain a point for future consideration, 

 and I shall only remark further, that the basalt of Staffa 

 will hereafter be shown to possess some characters giving 

 reason to suspect that it is composed of the same 

 materials. 



I am under the necessity of describing the next of 

 the overlying rocks found in this island under the 

 vague and general name of trap, though it exists in several 

 different forms. 



On the sides of Halival and Haiskeval it is found inter- 

 mingled with the augit rock in large masses, nor could I 

 after much research determine the precise nature of the 

 relation between them. The character of this trap varies 

 in different places. Sometimes it is a common greenstone, 

 at others a dark blue hard claystone, or a basalt, while 

 in still other instances it is porphyritic. In one place 



