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220 G. P. Scrape on Craters^ and the Liquidity of Lavas. 



intensity of the pressure by wliich it is confined Avhile yet within 

 tlie bowels of the earth? and would it not under these circum- 

 stances 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 tempera- 

 ture increased sufficiently, so as to give a certain degree of mo- 

 bility to these particles inter se^ and an imperfect li(^uidity to the 

 mass composed of them? And, supposing the mtumescence 

 thus occassioned to raise any portion of this seraidiquid matter 

 into the open air, would not the instantaneous absorption of cal- 

 oric from the contiguous particles, that must accompany the 

 vaporization of this water, and its escape in babbles or pores 

 and through cracks, owing to the nearly absolute cessation of 

 pressure, account for the sudden cooling down s^iid selfMlg^ or 

 consolidation, of the exposed surfaces, without having undergone 

 complete fusion (except in the case of mere superficial films), 

 notwithstanding their previous intense temperature, 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 por- 

 tions of many lava-streams, and their instantaneous consolidation 

 on exposure/ 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 absolute fusion; such, for example, as the 

 cracked and vitrified aspect of t^e feldspar-crystals of many tra- 

 chytes, the broken and dislocated appearance of the leuciteS; 

 feldspars, and otlo^er crystals in many basalts; the frequent ar- 

 rangement of their longest axes in the direction 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 spongj^, porous, and loosely crys- 

 talline texture of many trachytes, and their disposition in thick ^ 

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



imperfect 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 phenomena, have spoken of tlie vesicles or 

 air-bladders in lavas, as being proofs oi their having been in a 

 state of complete fusion, But have the loaves baked in our 

 ovens been in fusion? The comparison of a cellular scoria with 

 a loaf or a French roll, will show that vesicles of precisely simi- 



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