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Bafalt. 44* Vöigbt’s miftake in this inftance is certainly very excufable. He was led into it by the faulty defcriptions of alraoft all the obfervers of aclual volcanos; they almoft all miftake the floiving of lava for the fufion of lava, though its flowing proceeds almoft iolcly from the liquid bitumen with which it is mixed, as the flowing of mud proceeds from the water, and not from the li- quidity or diffolution of the earth it contains. Hcnce Mr. Dolomieu, the moft fag^cious, exad, and philofophical, of all of them,,allows, in the terms already quoted, that volcanic fire has not fufficient intenfity to vitrify cven the moft fufible fubftances : this, however, muft be adnutted with fome reftriäion, as vitreous lavas are fome- times found, though rarely, and fcoriae rnore abundantly ; but where any real fufion has taken place the veftiges of itare fufficiently difcermble. It has occurred to fome, that baialts and compaft lavas may have been in real fufion and yet by cooling flowly all marks of it may have been ob- literated, as happen, in the fufion of glals by Mr. Keir’s account; but Mr. Keir does not fav that all marks of fufion difappeared in bis expe- riment, but barely, that the glals cryftalhzcd and became femi-opake. I have fome now by me that cooled flowly in Mr. Dean s glafs houfe, part of which is cryftallized and femi-opake, but retains all the fharpnefs, luftre, and r.gidity of glafs; but the comparilon of ftony fubftances an glals is eflentially defedive j for, the falts that enter into the compofition of glafs evaporate in great meafure in a ftrong heac long continued, and leave the other ingredients nearly in their primitive ftate, and almoft as infufible ; whereas, in the fufion of moft ftones in our furnaces, nothing