362 GRANITE AND TRAP. [Ch. XXV. 



nating the volcanic from the plutonic rocks is sufficiently 

 great 5 for we must draw an arbitrary line between them, there 

 being an insensible passage from the most common forms of 

 granite into trap or lava. 



' The ordinary granite of Aberdeen shire/ says Dr. Mac- 

 culloch, ' is the usual ternary compound of quartz, felspar, and 

 mica, but sometimes hornblende is added to these, or the 

 hornblende is substituted for the mica. But in many places 

 a variety occurs which is composed simply of felspar and 

 hornblende, and in examining more minutely this duplicate 

 compound/ it is observed in some places to assume a fine grain, 

 and at length to become undistinguishable from the greenstones 

 of the trap family. It also passes in the same uninterrupted 

 manner into a basalt, and at length into a soft claystone, with a 

 schistose tendency on exposure, in no respect differing from 

 those of the trap islands of the western coast*.' The same 

 author mentions, that in Shetland a granite composed of horn- 

 blende, mica, felspar, and quartz, graduates in an equally 

 perfect manner into basalt f. 



It would be easy to multiply examples to prove that the 

 granitic and trap-rocks pass into each other, and are merely 

 different forms which the same elements have assumed according 

 to the different circumstances under which they have consoli- 

 dated from a state of fusion. What we have said respecting the 

 mode of explaining the different texture of the central and ex- 

 ternal parts of the Vesuvian dikes may enable the reader in some 

 measure to comprehend how such differences may originate J. 



The same lava which is porous where it has flowed over 

 from the crater, and where it has cooled rapidly and under 

 comparatively slight pressure, is compact and porphyritic in 

 the dike. Now these dikes are evidently the channels of com- 

 munication between the crater and the volcanic foci below ; so 

 that we may suppose them to be continuous to the depth of 

 several hundred fathoms, or perhaps two or three miles, or even 

 more ; and the fluid matter below, which cools and consoli- 



* Syst, of Geol,, vol. i. p. 157. f Ibid., p, 158. 



J See above, p. 124. 



