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 cannot 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
