1856.] SCROPE CRATERS AND LAVAS. 341 



reduced as to permit their expansion ; for instance, in the super- 

 ficial portions of a current ; and in some lava-currents throughout 

 the entire mass. 



The experiments of Mr. Knox, related in a paper read before the 

 Royal Society in 1824 *, had taught me that water in an appreciable 

 quantity is mechanically combined with the elementary particles of 

 all the crystalline rocks of igneous origin. The question, therefore, 

 arose, — Might not the water thus intimately disseminated through a 

 mass of crystalline lava, although at an intense temperature, remain 

 unvaporized, owing to the still greater intensity of the pressure by 

 which it is confined while yet within the bowels of the earth ? and 

 would it not under these circumstances exert an intense expansive 

 force upon all the confining molecular or crystalline surfaces between 

 which it lies, and thus occasion a tendency to separation among these 

 solid particles whenever the compressing forces were relaxed, or the 

 temperature increased sufficiently, so as to give a certain degree of 

 mobility to these particles inter se^ and an imperfect liquidity to the 

 mass composed of them ? And, supposing the intumescence thus 

 occasioned to raise any portion of this semi-liquid matter into the 

 open air, would not the instantaneous absorption of caloric from the 

 contiguous particles, that must accompany the vaporization of this 

 water, and its escape in bubbles or pores and through cracks, owing 

 to the nearly absolute cessation of pressure, account for the sudden 

 cooling down and setting, or consolidation, of the exposed surfaces, 

 without having undergone complete fusion (except in the case of 

 mere superficial films), notwithstanding their previous intense tem- 

 perature, amounting even to a white heat ? 



This supposition seemed to me to account satisfactorily, not only 

 for the absence of a vitreous texture even in superficial portions of 

 many lava-streams, and their instantaneous consolidation on expo- 

 sure, in cellular or porous slabs and cakes, but also for several other 

 characteristics of igneous rocks, not easily to be reconciled with the 

 idea of their having always issued from the earth in a state of abso- 

 lute fusion ; such, for example, as the cracked and vitrified aspect of 

 the felspar- crystals of many trachytes, the broken and dislocated 

 appearance of the leucites, felspars, and other crystals in many 

 basalts ; the frequent arrangement of their longest axes in the direc- 

 tion of the bed of the rock, that is, of the movement of the lava 

 when liquefied ; the finer grain often exhibited towards the tail or 

 extremity of a current than at its source, the brecciated lavas which 

 appear to have enveloped fragments in great number of the same 

 material without any fusion even of their finest angles. So also 

 might be explained the more or less spongy, porous, and loosely 

 crystalline texture of many trachytes, and their disposition in thick 

 beds or dome-shaped bosses, attesting their protrusion in a very im- 

 perfect state of liquidity, more resembling the intumescence of some 

 kinds of dough in an oven than the fusion of metal in a furnace. 



And here let me remark, that Dr. Daubeny, and some other 

 writers on volcanic phsenomena, have spoken of the vesicles or air- 



* Phil. Trans. 1825. 



