and the Nature of the Liquidity of Lavas. 193 
of the rise of aériform bubbles, as matter of a porous, pasty, or 
glutinous consistency, perhaps even semi-solid in texture and 
bulky in form. 
It might happen that, circumstances occasioning in turn the 
preponderance of the compressing over the expansive forces (by 
reason, for example, of a diminution of temperature), portions of 
the subterranean crystalline mass will, after a partial intumes- 
cence of the kind supposed, return to a state of solidity. The 
result may be a more fine-grained rock, owing to the partial dis- 
integration of the crystals; or, if the disintegration had pro- 
ceeded sufficiently far, new mineral combinations might take 
place. Indeed, Watt long since proved that the particles of 
even apparently solid rocks are capable, through changes in 
temperature, of internal motion sufficient to admit their re- 
arrangement according to polarity, that is, of crystallization. Still 
more likely is this result to occur on the condensation or escape 
of any fluid which had previously kept them from contact with 
each other, since the crystalline polarity can only exert itself 
within minute distances. And thus might be accounted for 
the frequently observed passages of granite and gneiss into 
syenite, greenstone, trap, or trachyte, and the varieties of mineral 
composition which these rocks at times exemplify. So also the 
transitions from the larger crystalline grain to the finer, and the 
dykes and veins which these rocks so often contain themselves, 
or intrude into their neighbours. So, too, the finer grain of the 
sides or selvages of such dykes might be owing to the greater 
disintegration of the crystals by friction along these sides as the 
matter was driven through them. 
Another problematical fact which this theory of an aqueous 
vehicle in heated granite would account for, is the usual appear- 
ance of the quartz in this rock, not in crystals, but as a paste or 
base, seeming to be moulded upon the crystals of felspar. Had 
the rock crystallized from a state of fusion, the felspar, being 
far more fusible than quartz, might have been expected to be 
the last, not the first, to crystallize. But if the water dissemi- 
nated through the rock were supposed to have taken the quartz 
into solution by aid of the alkalies present in the felspar, the 
fluid vehicle would in fact become a liquid or gelatinous silicate ; 
and upon consolidation would naturally mould itself on the fel- 
spar crystals, or appear as a paste to them. I adduced the 
hot siliceous springs of Iceland and other volcanic districts as 
peels that heated water under such circumstances could dissolve 
silex. 
Those who will take the trouble to refer to the 2nd, 4th, 5th, 
and 6th chapters of my ‘Considerations on Volcanoes,’ will see 
that the above is a brief summary of the arguments there put 
Phil. Mag. 8. 4. Vol. 14. No. 92. Sept. 1857. O 
