I 29. 



GRANITIC ERUPTIONS. 861 



29. Granitic eruptions. — In the Isle of Arran, the 

 granitic rocks were evidently erupted in a state of fusion, 

 for the slates are penetrated by veins of granite {Lign. 196, 

 Jig, 1) ; and in some instances are changed into fine-grained 

 mica, or hornblende slate. 



M. Dufrenoy describes granite veins traversing chalk, 

 in the Pyrenees, which have converted the cretaceous rock 

 into crystalline limestone, and generated in it veins of iron- 

 ore ; the following instructive fact is noticed by M. Elie de 

 Beaumont. In the environs of Champoleon, where granite 

 comes in contact with Jura limestone, whatever may be the 

 position of the surfaces in contact, the limestone and the 

 granite both become metalliferous near the line of junction, 

 and contain small veins of galena, blende, iron and copper 

 pyrites, &c. ; and at the same time the secondary rocks are 

 indurated and crystalline, while the granite has undergone a 

 contrary change.* Mr. Lyell mentions a remarkable ex- 

 ample of the alteration induced in stratified rocks by intru- 

 sions of syenite or granite ; near Christiania, in Norway, 

 very dark-coloured limestone is changed into white cry- 

 stalline marble, and slate into mica-schist ; traces of fossils 

 are not uncommon in some of the crystalline rocks, thus 

 unequivocally proving their metamorphic character. 



In Glen Tilt, in Scotland, schist and limestone are super- 

 imposed on and traversed by granite, and the latter is 

 intruded among the former rocks, and ramifies into innu- 

 merable veins in the most complicated manner, proving its 

 perfect fusion when erupted. f 



Granite never occurs stratified, but it often assumes a 

 laminar disposition, which may be considered as a modi- 

 fication of concretional structure. A prismatic or cuboidal 

 form is sometimes observable, but this appears to be the 



* Sir H. De la Beclie 



f See the highly valuable Memoir on the Geology of Glen Tilt, by 

 Dr. Macculloch, Geol. Trans, vol. iii. (first series) pp. 259-337. 



