Granite of Aberdeenshire. 37 



When the mixture of the two minerals, which forms the 

 greenstone of this granite, becomes minute, the rock is no 

 longer distinguishable from ordinary basalt; and, in some 

 specimens, it even appears that the felspar is at length entirely 

 excluded ; so that there remains nothing but that compact, yet 

 minutely granular aggregation of hornblende, which, according 

 to some mineralogists, constitutes the only genuine basalt. It 

 is further highly interesting to remark in this case, that these 

 basalts have often that internal concretionary structure which 

 causes them to exfoliate in laminse on exposure to air ; and 

 which is so remarkable a feature, not only in the basalts, but 

 in many of the greenstones of the trap family. I must also 

 observe, that among the rocks of this apparently simple cha- 

 racter, there are often found specimens which cannot be dis- 

 tinguished from the black clays tones which, by some authors, 

 are also called basalt, and which occur in such abundance in the 

 trap formation. In these, the peculiar lustre which characterizes 

 hornblende is absent ; the specimens presenting an uniformly 

 dull aspect, with an earthy fracture and a greater degree of 

 softness. 



Although, in speaking of these compounds, I have occasion- 

 ally used the terms greenstone and basalt, on account of their 

 accurate resemblance to those substances as they occur in the 

 trap family, and because these names are justified by the mi- 

 neral composition and character of the specimens, they must 

 still be considered as varieties of granite ; using that term, in 

 a general and geological sense, to comprise all the unstratified 

 rocks which are found beneath the primary strata, and which, 

 whatever differences they may present, are still associated by 

 some general mineral characters, and by a bond of mutual 

 transition. These terms, however, cannot be applied in this 

 case without great inconvenience ; and ought not to be used 

 hereafter in speaking of these substances, whenever the facts 

 now stated shall be admitted by geologists as established. I 

 have, in other writings, pointed out the great, and almost in- 

 corrigible confusion, which has already arisen, from applying 



