Mr Poulett Scrope's Considerations on Volcanos. 34<5 



these concretionary parts, if subsequently drawn out by the renewal of 

 motion in the lava, will give rise to veined, marbled, and brecciated rocks. 

 Numerous examples are given to show how completely these anticipations 

 accord with observation. 



2. A higher specific gravity, with an equally fine grain, increases the 

 fluidity of the lava and its extent of lateral spread ; the bubbles of vapour 

 will rise with greater force to the surface, which they will rend and break 

 up, leaving it bristling with asperities from the rapidity with which the 

 exposed surfaces congeal. Beneath this surface large cavernous blisters 

 will be frequent ; and the lower part of the current, on the contrary, very 

 compact. The great slowness with which this lower part congeals will 

 afford scope for the play of affinities, modified by the extreme fluidity 

 which it derives from its high specific gravity, the result of which, as is 

 shown in a subsequent chapter, will be tendency to the prismatic or co- 

 lumnar divisionary structure. The fine-grained basalts are specimens of 

 this variety. 



3. A coarse grain, coupled with a high specific gravity, by diminishing 

 the fluidity of the lava, increases the bulk or thickness of the beds into 

 which it is disposed, creates a porous texture, and a general dissemination 

 of rude angular cells. Such a mass will contract greatly on cooling, and 

 exhibit wide and numerous fissures of retreat ; by which the surface of 

 the current, particularly, will be shattered into rude flakes or angular 

 fragments- 



4. A lower specific gravity, together with a large crystalline grain, by 

 wholly preventing the vapour from uniting or ascending in bubbles, will 

 render the mass still more generally porous, and more bulky in figure ; as 

 is, in fact, the case with the earthy trachytes, lava sperone, piperno, &c. 



5. When the component crystals are still larger, that is less disintegrated, 

 nearly the whole of the vapour will be condensed by gradual cooling, with- 

 out much derangement in the position of the crystals, and the rock will, 

 therefore, be more compact and freer from pores. Some of the very large- 

 grained trachytes, dolerites, syenites, and granites may be taken as ex- 

 amples of this structure. If the crystals are non-conformably arranged, 

 the fluidity of the lava is at its minimum. If conformably, as in the 

 clinkstones, and other laminar crystalline rocks, the fluidity may be con- 

 siderable in the direction of the parallel plane surfaces of the crystals. 



6. Some masses of crystalline rock, the author supposes, may be occa- 

 sionally elevated in a solid state (by the expansion of lava at a great depth 

 beneath) without suffering any disintegration whatsoever, having either 

 been previously cooled down, or being preserved from ebullition by the 

 pressure of overlying strata, which are elevated together with them. Such 

 a circumstance would be in complete conformity with all the laws of sub- 

 terranean effervescence, and in the granitic axes of most mountain chains, 

 we recognize facts which can only be accounted for by such a mode of 

 production. 



The aqueous vapours that escape from most lavas, as they are consolidat- 

 ed, are accompanied by mineral substances, which become more and more 



