Of Diflinüfae Cbarafters. 4*3 6th, Vitrifaßiow; hyalite, otherwife called muller’s glafs, has pafled upon many asa produft of heat, the infufible at 150°; glittering frag- ments of felfpar bear often a deceittul refemblance t0 fth^'Porous lavas ; Amygdaloids with a trap bafis and argillites often refemble thefe fo ftrongty that Mr Sauflure, who found fome on the Alps, owns he would have taken them, if he met them elfewhere, for genuine lavas. 1 Sauft p. 142, 145 Mr. Doiomieu cautioufly marks this fource of delufion. “ Eafy as it may appear (fays he) “ todiftinguifh cellular lava, the obferver fhould ** be very circumfpeft, and examine with atten- « tion the local circumftances, beforc he can “ decide on the nature of a porous ftone he may « meet. External appearances may deceiye; “ there are rocks which by their decompofnion « acquire pores though they have no relation to « volcanos. I have leen ferpentines partly de- cayed by expofure to the air, in which round t< irregulär cavities were formed perfecrly refem- “ bling the pores of lavas. Some are capable of « forming the moft perfeft illufion. I found on “ the Pyrenees homblende fo altered by the “ viciflitudes of the atmofphere as to contain as “ many cells and cavernofities as certain blocks « launched from a volcano.” Ponces, 27 . Mr. Doiomieu, it is plam, was well awarethat fubftances found in countnes not volcanic cou d not owe their origin to volcanos. But unhappily he did not confider that all fubftances found m volcanic countries were not on that account ne- ceflarilv volcanic. Here lies the fource ot the numerous deceptions with which bis account of the Lipardi iflands abends. Simihtude and am>
