102 G. Poulett Scrope — On Lavas. 



Of course in these remarks I am referring solely to those lavas -whicli 

 after cooling exhibit a crystalline texture ; not to the vitreous lavas, 

 such as the obsidian and pumice streams of the Lipari Isles, Japan, 

 the Andes, Hawaii, and Bourbon, or the Pearlstones of Hungary and 

 and South America. The very fact of the existence of these vitreous 

 lava-flows — having a glassy texture not only on their surface but 

 throughout their mass — affords a negative proof that the stony and 

 crystalline lavas could not have been ejected in the same state of 

 complete vitreous fusion as the former, or why are they not equally 

 glassy and homogeneous throughout, having evidently cooled under 

 the same circumstances of exposure to the atmosphere? In the 

 Lipari Isles there are to be seen thick currents of obsidian and 

 pumice side by side with lavas, produced by the same volcano, com- 

 posed of an aggregation of interlaced crystals, chiefly labradorite, 

 as large as those of ordinary granite. A rock very similar to this 

 latter composes the mass called the Monte Olibano, near Puzzuoli, 

 seventy feet in thickness, which has flowed over beds of loose ashes 

 from the crater of the Solfatara into the sea. Though scoriaceous 

 near its surface, it is as largely crystalline there as in the interior of 

 the rock. 



But, it may be asked, if lavas are already crystalline when they 

 issue from a volcanic vent, how is their fluidity to be accounted for ? 

 I reply. Firstly, that lavas vary greatly in thi-s character; some, 

 though extremely viscous, having the fluidity of honey or of mud, 

 so as to flow rapidly down a steep slope, and spread, on moderately 

 flat surfaces, into wide and thin sheets ; others possessing such a low 

 degree of liquidity as to coagulate in thick beds (like that of Olibano 

 just referred to,) even on very steep slopes, and, when emitted upon 

 a nearly level surface, to accumulate in bulky hummocks over and 

 around the vent, such as the Puy de Dome and the neighbouring 

 trachytic bosses in Auvergne. It will be found on examination, 

 I believe, a general fact, that the more crystalline lavas show signs, in 

 their bulky forms and other circumstances, of their having been less 

 liquid when propelled from the vent than the more fine-grained ; 

 and also that the vitreous lavas exhibit marks of the greatest fluidity 

 — other circumstances, especially their specific gravity, being the 

 same. For it is certain, as might indeed be expected, that the 

 heavier basic or augitic lavas have generally spread further and in 

 thinner beds than the lighter, or acid (trachytic) ones. 



Secondly. So long ago as 1825, in the first edition of my Treatise 

 on Volcanos, I suggested that the fluidity of lavas, even when com- 

 posed for the most part of ready-formed crystals or granules, might 

 be accounted for by the presence, throughout the mass, of a certain 

 amount of interstitial water — of course at the temperature of the 

 lava itself, and therefore probably in the state of minute globules, 

 such as the experiments of M. Boutigny exhibited, and tending to 

 flash into steam on the least relaxation of the pressure upon them. 

 Hence the vesicles and air cells that form in the upper portions of 

 lava streams, and often burst from them in jets of vapour. Hence, 

 too, the instantaneous cooling and consolidation of these exposed 



