302 
Mr. Watt’s Observations 
appear, that the transition from the stony to the porphyrytic 
state is rapid, for perfectly homogeneous lavas are among the 
rarest of volcanic products. The porphyritic lavas are most 
numerous ; and it is needless to detail the varieties they present. 
But, though the process of arrangement has often only advanced 
thus far, it has in many instances proceeded much farther, and 
it is by no means unusual to find the entire basis regularly 
arranged into crystalline bodies ; thus, to cite a well known 
instance, in many of the ancient lavas of Somma, large augites 
are imbedded in a crystalline mass, formed of minute crystals of 
leucite, together with another crystalline substance, whose na- 
ture is not perfectly determined. 
The casual occurrence of volcanic glass is nowise at variance 
With this account, as it is sufficiently probable, that some glasses 
may have a much greater tendency to crystalline arrangement 
than others possess; and it cannot appear extraordinary, that 
regular crystals should sometimes be generated, even in the 
glass, as it is a matter of daily occurrence in artificial glasses, 
and in furnace slags. 
If the distinction attempted to be shown between igneous 
fusion and solution be established, it may offer a means of ac- 
counting for the abundance of peculiar bodies in lava, which do 
not exist in other situations, or at least are of extremely rare 
occurrence. For, if the igneous action decomposes the molecules 
of the substances on which it operates, there seems every 
“ frrgments, as glass properly does. Besides, it has I know not what of unctuosity to 
“ the touch, and to the eye, which is not perceptible in the more perfect volcanic 
** glasses. It yields sparks with steel, like the lava; but the lava is wholly opaque, and 
•* the glass, at the angles and thin edges, has considerable transparency. It is qnly 
opaque where the globules are, which appear to be particles of lava,” 
