Ch. XXV.] 



GRANITES OF DIFFERENT AGES. 357 



assumes an appearance exactly resembling that of horn-stone. 

 The associated argillaceous schist often passes into hornblende 

 slate, where it approaches very near to the granite *. 



In the plutonic, as in the volcanic rocks, there is every 

 gradation from a tortuous vein to the most regular form of a 

 dike, such as we have described as intersecting the tuffs and 

 lavas of Vesuvius and Etna. In these dikes of granite, which 

 may be seen, among other places, on the southern flank of Mount 

 Battoch, one of the Grampians, the opposite walls sometimes 

 preserve an exact parallelism for a considerable distance. It is 

 not uncommon for one set of granite veins to intersect another, 

 and sometimes there are three sets, as in the environs of 

 Heidelberg, where the granite of the right bank of the Rhine 

 is seen to consist of three varieties differing in colour, grain, and 

 various peculiarities of mineral composition. One of these, 

 which is evidently the second in age, is seen to cut through an 

 older granite, and another, still newer, traverses both the 

 second and the first. These phenomena were lately pointed 

 out to me by Professor Leon hard at Heidelberg. 



In Shetland there are two kinds of granite. One of these, 

 composed of hornblende, mica, felspar, and quartz, is of a dark 

 colour, and is seen underlying gneiss. The other is a red 

 granite which penetrates the former everywhere in veins y. 



Granites of different ages. It was formerly supposed that 

 granite was the oldest of rocks, the mineral product of a par- 

 ticular period or state of the earth formed long antecedently to 

 the introduction of organic beings into the planet. But it 

 is now ascertained that this rock has been produced again 

 and again, at successive eras, with the same characters, pene- 

 trating the stratified rocks in different regions, but not always 

 associated with strata of the same age. Nor are organic remains 

 always entirely wanting in the formations invaded by granite, 

 although their absence is more usual. Many well authenticated 

 exceptions to the rule are now established on the authority of 



* Macculloch, Geol. Trans., vol. iii. p. 259. 

 t Macculloch, Syst. of Geol., vol. i. p. 58. 



