Granite of Aberdeenshire. 4$ 



argillaceous earths, are in both cases, converted into substances 

 resembling chert. Where, on the contrary, they consist of 

 carbonate of lime alone, they assume a crystalline texture near 

 the points of junction with the veins ; and the causes of that 

 change are very obvious in those instances where, in the distant 

 portions of the rocks, these limestones have a compact or earthy 

 texture, as is particularly the case where chalk is traversed 

 by trap veins. If, in other cases, the veins of granite produce 

 less effect on the adjoining rocks than those of trap, it must 

 be recollected, that the former traverse exclusively the primary 

 strata, of which the mineral characters are such as to be scarcely 

 capable of undergoing those changes which, in the contact of 

 trap with the secondary strata of softer texture, are easily in- 

 duced. 



On this subject of the general analogy between these two 

 classes of veins, it may lastly be remarked, that where granite 

 ramifies into minute filaments, the mixed crystalline texture 

 disappears, and the ultimate branches become uniformly com- 

 pact, appearing to consist of an intimate mixture of quartz and 

 felspar, or of felspar alone. In the same manner, where trap 

 veins have been found ramifying in the same way, the minute 

 branches lose the crystalline character and acquire a fine com- 

 pact texture, so as to resemble either pitchstone, or that siliceous 

 schist which is called Lydian stone. Such ramifying veins 

 it is true, are rare, but they have been pointed out, in that 

 work on the Western Islands already mentioned, in several 

 plates, namely, in Barra, South Uist, and Sky. 



To enter further into this subject, and to support it by all 

 the evidence of facts which might be brought forward for that 

 purpose, would be to involve the whole history of these two 

 remarkable and extensive classes of rock, and, in fact, to pro- 

 duce a treatise utterly incompatible with the nature of this com- 

 munication^ and with the space to which it is unavoidably 

 limited. Practical geologists will be at no loss to supply, 

 from their own knowledge, whatever is wanting ; and those to 

 whom geological investigation is yet new, or who have suffered 

 themselves to be diverted from the study of nature by hypo- 



