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218 G. P. Scrope on Craters, and the Liquidity of Lavas. 



it that thej do not all present the same glassy texture which is 

 seen in somCj the obsidianSj pitchstones, and puniiceous layas es- 

 peciallyj and in the ropy, cavernouSy filamentous basalts of Ki- 

 lauea, Iceland, and Bourbon, and which these very crystalline 

 and stony lavas themselves put on when melted under tne blow- 

 pipe or m 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 arc cited in support of this assertion. In the present 

 day, probably the process by which Messrs, Chance & Co., of 

 Birmingham, de vitrify a mass of fused basalt (fi'om 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 feet more certain than th^s,- 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 

 instantaneously, 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 scori- 

 form crust, or the solid cakes and slabs which so instantly form 

 upon every exposed surface of lava, nay, even the scoria which 

 is 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 sep- 

 arating 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 in which the basalt used there (and which is the same 

 upon which Mr. Gregory Watt experimented) conducts itself in 

 their furnaces, and I found, that when the liquid and fused con- 

 tents of a furnace at a white heat is poured out upon a brick or 

 other floor into the open air, so as to represent a stream of lava 

 flowing out of a volcanic vent, it consohdates throughout, what- 

 ever its bulk, into a homogeneous and purely vitreous black ob- 

 sidian, in fact, an absolute glass, with a conchoidal fracture and 

 sharp cutting edges. It is only when made to consolidate very 

 slowly in an oven kept at a high temperature for some days, 

 that it assumes the deadened and semi-crystalline texture of the 

 manufactured article. 



If this process be interrupted, it is found to have commenced 

 by the formation, at numerous points within the vitreous mass, 

 of globular concretions about the size of a small pea, of a lighter 

 color than the base, and having a pearly lustre and radiated 



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