Ch. XXXIIL] rocks altered by granite veins. 709 



learn — that the presence of these elements in certain proportions is 

 more favorable than in others to their assuming a crystalline or true 

 granitic structure ; but it is also ascertained by experiment, that the 

 same materials may, under different circumstances, form very different 

 rocks. The same lava, for example, may be glassy, or scoriaceous, or 

 stony, or porphyritic, according to the more or less rapid rate at 

 which it cools; and some trachytes and syenitic-greenstones may 

 doubtless form granite and syenite, if the crystallization take place 

 slowly. 



It has also been suggested that the peculiar nature and structure of 

 granite may be due to its retaining in it that water which is seen to 

 escape from lavas when they cool slowly, and consolidate in the atmos- 

 phere. Boutigny's experiments have shown that melted matter, at a 

 white heat, requires to have its temperature lowered before it can 

 vaporize water ; and such discoveries, if they fail to explain the manner 

 in which granites have been formed, serve at least to remind us of the 

 entire distinctness of the conditions under which plutonic and volcanic 

 rocks must be produced.* 



It would be easy to multiply examples and authorities to prove the 

 gradation of the granitic into the trap rocks. On the western side of 

 the fiord of Christiania, in Norway, there is a large district of trap, 

 chiefly greenstone-porphyry and syenitic greenstone, resting on fossil- 

 iferous strata. To this, on its southern limit, succeeds a region equally 

 extensive of syenite, the passage from the volcanic to the plutonic 

 rock being so gradual that it is impossible to draw a line of demar- 

 cation between them. 



" The ordinary granite of Aberdeenshire," says Dr. MacCulloch, " is 

 the usual ternary compound of quartz, felspar, and mica ; but some- 

 times 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 ob- 

 served 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 

 hornblende, mica, felspar, and quartz graduates in an equally perfect 

 manner into basalt.f 



In Hungary, there are varieties of trachyte, which, geologically 

 speaking, are of modern origin, in which crystals, not only of mica, 

 but of quartz, are common, together with felspar and hornblende. 

 It is easy to conceive how such volcanic masses may, at a certain 

 depth from the surface, pass downwards into granite. 



* E. de Beaumont, Bulletin, vol. iv., 2e ser., pp. 1318 and 1320. 

 f Syst. of Geol., vol. i. pp. 157 and 158. 



