424 Second Appendix logy to real and. acknowledged lavas, or at leaffc fufibility, enters as neceflarily into the proof of the volcanic origin of any foffil as the mere cir- cumftance of locality. It is the union of both that rnakes tne proof complete. Mr. Faujas, whofe teftimony on this occafion is above exception, acknowledges that traps and porphyries may afiume the appearance of lavas, fo completely, as to lead the moft experienced obfervers into error, if local circumftances be not attended to. It is by attention to them that he has Ihewn that neither the toadftone of Derby- Ihire, nor the whinftone of Scotland, are teally volcanic *. Dr. Prieftley, among his numerous difcoveries, feems to have found a decifive charadter of lavas: he obferved that real lavas yidd but little air, -an ounce not afl'ording above 5,5 ounce meafures of air, and fometimes fcarce any. Whereas fiditious lavas, as bafalts, &c. yield from 20 to 20 ounce meafures. 6 Pri. 217. I have often found real lava to yield hepatic air, bafalt never. Some ftones which confefledly have been formed by means of water, have neverthelefs had a vol canic origin afcribed to them. They have been deduced from muddy eruptions, of which feveral are faid to have iftued from Vefuvius in the laft Century. Signior Gioeni, however, one of the lateft and moft judicious obfervers of that vol- cano, fatisfadloriiy refutes this opinion. 1 hat water flrould be colledted in the focus of an adtive volcano, is indeed incredible. Some may indced be formed during the inflammation, but in a * Sur les Traps, 23, 46, 62. quantity
