on Basalt , &c. 
$01 
all the molecules have been permitted to arrange. Those granites 
called porphyritic, in which large crystals of feldspar are im- 
bedded in a basis compounded of the ordinary ingredients of 
granite in small grains, are apparently generated from a men- 
struum in which the molecules of one species, being greatly 
predominant in number to the rest, are the first to exercise their 
polarity, and constitute large crystals, which are afterwards sur- 
rounded by smaller ones* resulting from the successive separa- 
tions of the remaining elementary molecules. 
The changes of the substance that led to the foregoing re- 
marks, serve to show that they are not altogether hypothetical ; 
and any proof that may appear deficient, seems to be provided 
by the phenomena exhibited by lavas, in which may be observed 
every step of the passage from the vitreous to the stony, from 
that- to the porphyritic, and finally to the granitic state. The 
lava of Lipari, which passes from glass to lava, by the generation 
of minute globules, may be cited, on the authority of Spallan- 
zani, as an instance of the commencement of the process of 
arrangement ;* and, were not their origin still disputed, I might 
also cite the pitchstone lavas of the Euganean hills. It would 
* Spallanzani, Viaggi alle due Sicilie. Tomo Secondo , page 238. The whole 
passage, literally translated, stands thus. “ This lava has a basis of feldspar, of a fine 
« and compact grain, a splintery fracture, rough to the touch, and emitting sparks, 
« like flint, when struck with steel. It has an ash colour, in some places approaching 
“ to a leaden colour. It is thickly filled with an immensity of little bodies, which 
“ would be distinguished with difficulty, from the resemblance of their colour to that 
“ of their basis, were it not for their globular form. But this lava is joined to a great 
** mass of glass, which forms a whole with it, without any division or separation 
“ between them ; and this lava, which in many places retains its own nature, is in 
** many other places reduced to glass. Some parts of this glass are filled with the 
“ same little bodies, but other parts are pure glass. This is in general very compact, 
« has a dead black colour, and breaks rather into irregular pieces than into undulated 
