OBSERVATIONS ON BASALT, &C. \QQ 



flruum in which the molecules of one fpecies, being greatly 

 predominant in number to the reft, are the firft to exercife their 

 polarity, and conftitute large cryllals, which are afterwards fur- 

 rounded by fmaller ones, refuking from the fucceffive fepara- 

 tions of the remaining elementary molecules. 



The changes of the fubftance that led to the foregoing re- The pafLgc 

 marks, ferve to (how that they are not altogether hypothetical ; from thc v 'treous 

 and any proof that may appear deficient, feemsto be provided from that to the' 

 by the phenomena exhibited by lavas, in which may be obferved porphyrinic, and 

 every ftep of the paflage from the vitreous to the ftony, from ^ti/ftate are™* 

 that to the porphyritic, and finally to the granitic Hate. The obfervable in 

 lava of Lipari, which pafles from glafs to lava, by the genera- avas * 

 tion of minute globules, may be cited, on the authority of Spal- 

 lanzani, as an inftance of the commencement of the procefs of 

 arrangement ; * and, were not their origin ftill difputed, I might 

 alfo cite the pitchftone lavas of the Euganean hills. It would 

 appear, that the tranfition from the fiony to the porphyry lie ' 

 irate is rapid, for perfectly homogeneous lavas are among the 

 rareft of volcanic produces. The porphyritic lavas are moft 

 numerous ; and it is needlefs to detail the varieties they prefent. 



* Spallanzani, Viaggi alle due Sicilie. Tomo Secondo, page 238. 

 The whole pafTage, literally tranflated, ftands thus : " This lava 

 « has a bafis of feldfpar, of a fine and compact grain, a fplintery 

 *■' fracture, rough to the touch, and emitting fparks, like flint, 

 '* when ftruck with fteel. It has an afh colour, in fome places ap- 

 " proaching to a leaden colour. It is thickly filled with an immen- 

 «« fity of little bodies, which would be diftinguifhed with difficulty, 

 " from the refemblance of their colour to that of their bafis, were 

 " it not for their globular form. But this lava is joined to a great 

 " mat's of glafs, which forms a whole with it, without any divifion 

 " or feparation between them ; and this lava, which in marry places 

 f' retains its own nature, is in many other places reduced to glafs. 

 " Some parts of this glal's are filled with the fame little bodies, but 

 " other parts are pure glafs. This is in general very compact, has 

 " a dead black colour, and breaks rather into irregular pieces than 

 " into undulated fragments, as glafs properly does. Befides, it has 

 " I know not what of un&uofity to the touch, and to the eye, 

 " which is not perceptible to the more perfect volcanic glaffes. It 

 «« yields fparks with fteel, like the lava; but the lava is wholly 

 f* opaque, and the glafs, at the angles and thin edges, has confider- 

 «' able tranfparency. It is only opaque where the globules are, 

 " which appear to be particles of lava." 



But, 



