444 PLADDA. GEOLOGY. 



prevents it from being classed with this rock; although 

 it is in many cases impossible to determine the boundaries 

 of this species, if that may be called a species which 

 is in a state of continual fluctuation, and is occasionally 

 found passing into every other modification ranked under 

 the comprehensive term of trap. It will be more safe 

 to consider it as consisting principally, perhaps in some 

 cases entirely, of an indurated claystone highly charged 

 with protoxide of iron. If there be a darker substance 

 intermixed, it is too minutely divided to be ascertained 

 in a satisfactory manner by the eye ; nor do these rocks 

 admit of that mechanical analysis by trituration and 

 washing, which has in some cases been successfully 

 applied to mixed substances. But it is unnecessary now 

 to enter into minute details respecting these rocks. 

 To distinguish the variations which they undergo, even 

 in so small a space as that which they occupy here, 

 would lead to protracted descriptions from which no 

 advantage could result to science, and no interest be 

 derived in a topographical view.* 



* The nature of basalt is a frequent source of difficulty, and must 

 necessarily remain so till some acknowledged authority shall decide on a 

 limited distinction ; if one is necessary. The term seems at present 

 applicable to the black or very dark indurated claystones, or clinkstones, 

 to intimate mixtures, of augit, or of hornblende, with the same substances, 

 and, apparently also, to hornblende alone when of a very fine granular 

 and condensed texture. 



On re-examining a fresh and more extensive collection of the rock of 

 Staffa, 1 am in doubt if its nature was correctly stated in the preceding 

 article on that island. The specimens in question, at least, seem to be a 

 uniform substance, of one ingredient only, which is a granular splin- 

 tery material resembling clinkstone, highly coloured with iron, but of a 

 greenish black hue. 



