Volcanos. 1 25 



duced; and 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 obser- 

 vation. 



"2. A higher specific gravity, with an equally fine grain, in- 

 creases the fluidity of the lava and its extent of lateral spread - r 

 the bubbles of vapour will rise with greater force to the surface, 

 which they will rend and break up, leaving it bristling with as- 

 perities from the rapidity with which the exposed surfaces con- 

 geal. 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 gra- 

 vity, the result of which, as is shown in a subsequent chapter, will 

 be tendency to the prismatic or columnar 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 thick- 

 ness of the beds into which it is disposed, creates a porous texture, 

 andji 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 particular- 

 ly, will be shattered into rude flakes or angular fragments. 



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

 grain, by wholly preventing the vapor 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 tra- 

 chytes, lava sperone, piperno, &c. 



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

 disintegrated, nearly the whole of the vapour will be condensed 

 by gradual cooling, without 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, do- 

 lerites, syenites, and granites may be taken as examples of this 

 structure. If the crystals are non-conformably arranged, the flu- 

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

 clinkstones, and other laminar crystalline rocks, the fluidity may 

 be considerable in the direction of the parallel plane surfaces of 

 the crystals. 



" 6. Some masses of crystalline rock, the author supposes, may 

 be occasionally elevated in a solid state, (by the expansion of 

 lava at a great depth beneath,) without suffering any disinte- 

 gration whatsoever, having either been previously cooled down, 

 or being preserved from ebullition by the pressure of overlying 



