1856.] SCROPE — CRATERS AND LAVAS. ^139 



seotly advert, led me to an opinion expressed in the works above 

 referred to, that the ordinary crystalline or granular lavas (making 

 exception of the vitreous varieties), although at a white heat at the 

 moment of their emission from a volcanic vent, are not in a state of 

 complete fusion ; that a large proportion, at least, if not all, of the 

 crystalline or granular particles of which, when cooled and consoli- 

 dated, they appear composed, are already formed and solid, their 

 mobility being aided by the intimate dissemination through the 

 mass of a minute but appreciable quantity of some fluid, — in all pro- 

 bability water, — which is prevented from expanding wholly into 

 vapour by the pressure to which it is subjected while within the 

 volcanic vent, or in the interior of the current, until that pressure is 

 sufliciently reduced to allow of its expansion in bubbles, or its escape 

 through pores or cracks, by which it passes into the open air from 

 the surface of the intumescent lava. 



I was strengthened in this opinion by several concurrent consi- 

 derations : — 



1. If all lavas are (as they are usually supposed to be) in a state 

 of complete fusion when they issue from a volcano, how is it that 

 they do not all present the same glassy texture which is seen in 

 some, the obsidians, pitchstones, and pumiceous lavas especially, and 

 in the ropy, cavernous, filamentous basalts of Kilauea, Iceland, and 

 Bourbon, and which these very crystalline and stony lavas them- 

 selves put on when melted under the blowpipe or in a furnace ? The 

 usual answer is, that the granular and crystalline texture is acquired 

 subsequently to emission by slow cooling ; and the experiments of 

 Gregory Watt and Sir James Hall are cited in support of this 

 assertion. In the present day, probably the process by which 

 Messrs. Chance and Co., of Birmingham, devitrify a mass of fused 

 basalt (from the Rowley rag, near Dudley) by causing it to cool 

 slowly in an ''annealing furnace," would be considered as a strong 

 confirmatory fact. 



But there is no fact more certain than this, that the superficial 

 portions, at least, of a lava-current flowing in the open air, do not 

 cool slowly. On the contrary, they are rapidly, I might say instan- 

 taneously, upon their exposure, consolidated and cooled down to a 

 temperature which permits them to be handled and even walked 

 upon without damage. How is it that this scoriform crust, or the 

 solid cakes and slabs which so instantly form upon every exposed 

 surface of lava, nay, even the scorisR which are tossed up in a liquid 

 state by the eruptive jets, and harden while yet in the air before 

 they fall, exhibit on fracture no glassy texture, but much the same 

 earthy or stony grain, and occasionally crystals of considerable size 

 in the solid matter separating their cellular cavities, as is found in 

 the interior of the current which is known to have cooled very 

 slowly ? How is it that some lava currents are stony throughout, 

 others vitreous throughout, as, for example, some of the large 

 pumice-streams of Lipari, Iceland, and the Andes ? 



I have recently visited the manufactory of the Messrs. Chance, at 

 Oldbury, near Birmingham, for the purpose of examining the mode 



