APPENDIX. 



385 



the earth, generally issue from their flanks. The longest known cur- 

 rent of modern lava is in Iceland — it extends sixty miles ; but the vol- 

 canoes in that island bear no proportion to the magnitude of the lunar 

 volcanoes. 



Geologists who are reluctant to admit the extensive agency of fire 

 on the surface of the terrestrial globe, would have their difficulties re- 

 moved, were they to study attentively the surface of the moon with a 

 powerful telescope ; for there we see the entire hemisphere of a plan- 

 etary body subjected to the agency of volcanic fire^ 



Since my return from the extinct volcanoes of Auvergne, I hate 

 frequently amused myself in comparing the structure of parts of the 

 moon's surface with that of the volcanic districts in central France ; 

 and I could scarcely avoid the conclusion^ that the summits of many 

 volcanic mountains in the moon, which reflect so much more light 

 than the other partsj are, like those in Auvergne, composed of rocks 

 analogous to white pumice or trachyte. I have suggested these hints* 

 to direct the attention of geologists and astronomers to our attendant 

 planet. Is it inhabited ? Is it passing to a habitable state ? or does: 

 it present the ruins of a former habitable globe, torn by the power- 

 ful agency of volcanic fire ? Its appearance seems most to agree with 

 the latter condition. Perhaps the perfection to which telescopes are 

 advancing on the Continent may enable astronomers, at no distant pe- 

 riod, to answer these questions. 



ORBICULAR PORPHYRY AND ORBICULAR GRANITE OF CORSICA. 



These are two of the most rare and beautiful rocks ; but little is 

 known respecting their relation with other rocks in that island. Ac- 

 cording to specimens of considerable size, which I have before me, 

 this porphyry is composed of compact felspar, varying in colour from 

 a greenish to a reddish brown. The globules vary in diameter from 

 one third of an inch to three inches. The most perfectly formed glob- 

 ules have a small globule in the centre of each, from which ranges of 

 minute globules diverge, giving to the large globules the appearance of 

 a radiated diverging structure, more or less regular. In the smaller 

 globules there are concentric circles, which disappear in the larger 

 ones, except near their superficies. The paste in which these glob- 

 ules are imbedded contains also minute globules of lighter coloured 

 felspar, variously arranged. The larger globules are some of them 

 elongated, as if they had been in fusion. The experiments of Mr. G. 

 Watt on basalt (see page 146) elucidate the formation of orbicular por- 

 phyry. 



The globular structure was probably developed during the semili- 

 quefaction of the mass, which formed globules, instead of perfect crys- 

 tals, as in common porphyry. The globules in the Corsican porphy- 

 ry can be easily detached from the mass. Common porphyry, in 

 which the imbedded felspar occurs in rounded spots, is called Vario- 

 lite. 



The orbicular granite of Corsica is better know in this country : it 

 IS a finely granitic rock, composed of white felspar and blackish greeii 



49 



