Ch. III.] COMPARED WITH THOSE OF CORNWALL. 25 



characteristic mineral ; but, with this difference, the resem- 

 blance will be found very great. 



For example, if we examine the granitic range of Aberdeen- 

 shire, common or perfect granite will be found in the moun- 

 tains at the sources of the Dee; as also the coarse-grained, 

 fine-grained, and porphyritic varieties of the same rock : but 

 hornblende is generally present, forming hornblendic granite. 

 This mineral often takes the place of the mica altogether, 

 giving rise to a perfect syenite, which exhibits numerous 

 varieties, according to the size of the felspar, quartz, and 

 hornblende, and the proportion in which these substances are 

 united. 



As in the case of shorl-rock in Cornwall, so in this syenite, 

 one of the minerals sometimes disappears : in the former, this 

 is the felspar ; but in the latter, the quartz. And when this 

 happens, the compound cannot be distinguished from some 

 greenstones or traps, which are associated with the stratified 

 rocks. The geologist just mentioned thus describes * the 

 different kinds of this hornblende-rock : " These rocks are 

 fundamentally composed of felspar and hornblende ; and ac- 

 cording to the magnitude of their parts, and the relative 

 proportions of these ingredients, the appearances of the 

 specimens vary. In some rare instances, the crystals of 

 hornblende are so large as to attain half an inch in length, 

 although they are not defined in form ; and as the felspar is 

 commonly white, this variety forms beautiful specimens. 

 From this size, the portions of each mineral vary in grada- 

 tion ; forming compounds, which resemble the coarser and 

 finer greenstones. The hornblende is invariably black, but 

 it is not always intermixed in an uniform manner with the 

 felspar ; some instances occurring in which, to the general 

 indiscriminate mixture, are superadded large and distinct 

 patches of irregular crystals, producing that appearance 

 which, when it takes place in ordinary granite from a similar 



* Quarterly Journal of Science and the Arts, vol. x. p. 36. et seq. 



