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Observations on Basalt, and on the Transition from the vitreous to the 

 stony Texture, which occurs in the gradual Refrigeration of melted 

 Basalt ; with some geological Remarks. In a Letter from Gregory 

 Watt, Esq. to the Right Hon. Charles Greville, V.P.R.S. Read 

 May 10, 1804. [Phil. Trans. 1804, p. 279.] 



The number of conjectures that have been formed respecting the 

 nature of basalt, and the variety of opinions hitherto entertained 

 concerning its aqueous or volcanic origin, not having yet proved sa- 

 tisfactory to those who are more seriously bent upon the discovery 

 of truth than the triumph of victory in a controversy ; we are much 

 indebted to the author of this paper for the ample body of obser- 

 vations and ingenious remarks it contains, from which, though they 

 by no means establish either of the contested hypotheses, much may, 

 however, be collected, which pannot fail to throw additional light 

 on various intricate points in mineralogy and geology. 



The main object here agitated, is the transition from the vitreous 

 to the stony texture, which takes place in the refrigeration of glass ; 

 and the following experiment takes the lead in this investigation, 

 and affords the materials for the subsequent disquisitions. 



About 7 cwt. of the kind of basalt called Rowley Rag, of which a 

 minute description is here given, were put into a common reverbera- 

 tory furnace, strongly heated for several hours. It soon melted with 

 a less degree of heat than would have fused an equal weight of pig 

 iron, and subsided into the deeper part of the furnace in the form of 

 a liquid but rather tenacious glass. A portion of it, on being taken 

 out and suffered to cool, retained the character of perfect glass. But 

 the remainder of the mass was left in the furnace, which was gra- 

 dually cooled, and in eight days it was extracted, being cold on its 

 surface, but still retaining a considerable degree of internal heat. Its 

 shape being very irregular, it was so differently affected by the same 

 degree of heat, and the same progressive refrigeration, that no uni- 

 formity of texture could be expected. This circumstance, however, 

 which might have been obviated by care, is not to be regretted, since 

 it has fortuitously disclosed the singular peculiarities in the arrange- 

 ment of bodies passing from a vitreous to a stony state, which are 

 the chief object of this paper. 



A very minute description of this mass is next given, both in its 

 progress towards vitrification, and in its subsequent refrigeration. 

 What appears most relevant is, that in the fluid glass a tendency 

 towards an arrangement of particles is first developed by the for- 

 mation of minute globules, thickly disseminated throughout the mass. 

 In the process of cooling, these globules adapt their form to their 

 confined situation, gradually filling up every interstice ; and they 

 finally assume the appearance of a substance apparently homoge- 

 neous, equally unlike glass, and the parent basalt, but much resem- 

 bling some varieties of jasper in the compactness of its texture, and 

 in its opacity. 



If the temperature adapted to the further arrangement of the 



