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Bafalt. 437 half of the whole, or conchoidal and glazed if it extended farther, and fome air blebs, even if converted into glafs ; therefore bafalt wants the peculiar marks of fufion. To this plain reafon feveral anfwers have been given. According to Mr. Hamilton, all the ba- l'alts, he has feen, exhibit, in one part or other of their fubftance, air-holes ; and it is remarkable (fays he), that even the pillars of our Giant’s Caufeway, which are fingularly compaft, have their upper joints mors or lefs excavated *; and for this he appeals to a view of thofe of Dublin (College. The grofs pillars in the l capes and mountains, he teils us, frequently abound in thofe air-holes, through all their parts, which fome- times contain clay, and other foreign bodies, and the irregulär bafalts where the pillars ceafe, or which lies over them, is, in general, extiemely honey-combcd, containing in its cells cryftals of zeolyte, brown clay, or üeadtes, &c. After reading this pafläge, I carefully exa- mined the pillars in our College, and found, in truth, that in fome part, namely, in the top ot each pillar, there were two or threc minute cavi- ties of the fize of the head of a pin, and one twenticth of an inch, perhaps, in depth, and no more, in the fpace of twenty fquare feer. It is plain therefore, thefe arofe from decompofition, or external accidents; for, if they proceeded from internal air, they would have penetrated niuch deeper into the body of the ftone. lhis Mr. Dolomieu acknovvledges, and gives even as a criterion of ftoncs rcally volcanic, <c I pori eljftono nell’ interno equalmente inquelle pietre * P. 149. F f 3 ** cbt 1