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3J>° Aggregates. Chy. An. 1790, 16. Very frequently it is dif- integrated or decompofed. It is faid to differ in hardneis and fufibility from common felfpar; thofe moft ulual in granites poffeffing thefe pro- perties in a higher degree; it may therefore be, if groy, the labradore; if browniß: red, the petri- lite ; if greeniß, the felfite; porphyries containing large cryftals or fragments of felfpar, from 0,4 to 0,9 of an inch, are by the French called fer- pentines; their bafis is fuppofed to be a jafper, but commonly, I beiieve, it will be found to be a hornftone. Briffon, p. 126. 35 Rozier 128. 'l he fpecific gravity of porphyries is necef- farily different with the nature and proportion of eir bafes. OfSiliceous’Porphyries. Lefke S. 1090. Siliceous porphyries have either jafper, horn ftone, pitchftone, obfidian, filiceous fhiftus, ihif- tofe hornftone, or felfpar itfelf for their bafis. Jafper Porphyry. The exiftence of this has lately bccn called i» queftion. Mr. Werner, exprcflly fays, he never met any. 1 Bergm. Journ. 1789,600, inanote. Mr. Faujas, from the great fufibility of the por- ( phyries, that were prefumed to have jafper for their bafis, thinks that.it muft be of a different nature, This point, however, is not yet per- fe£Hv afcertained; fee Lefke G. 212. and 2. Chy. An. 1790, 19. nor perhapscan, in p» r ' ticular inftances, without examining the fufibility of the fpecimens in queftion, Hornftone
