Ch. XXXIH] geanite into teap. 565 



The minerals -which constitute alike the granitic and volcanic rocks 

 consist, almost exclusively, of seven elements ; namely, silica, alumina, 

 magnesia, lime, soda, potash, and iron (see Table, p. 475) ; and these may 

 sometimes exist in about the same proportions in a porous lava, a compact 

 trap, or a crystalline granite. It may perhaps be found, on further ex- 

 amination — for on this subject we have yet much to learn — that the pres- 

 ence 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 differ- 

 ent circumstances, form very different rocks. The same lava, for example, 

 may be glassy, or scoriaceous, or stony, or porpkyritic, according to the 

 more or less rapid rate at which it cools ; and some trachytes and sye- 

 nitic-greenstones may doubtless form granite and syenite, if the crystal- 

 lization 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 atmo- 

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

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

 pourize 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 fossilife- 

 rous 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 demarcation be- 

 tween 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 observed in 

 some places to assume a fine grain, and at length to become undistin- 

 guishable 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 

 basal t.f 



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



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

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



