VOLCANO. 



dance at TenerifFe, Kamtfchatka, and various volcanic 

 countries ; but it is by np means fo common a produft as 

 iloiiy lava. 



The objcftions to the volcanic origin of obfldian, founded 

 on its lofs of colour, and its tumefaftion at a low degree of 

 heai, are deprived of their force by the dilcoveries of fir 

 James Hall before mentioned. Thefe experiments prove, 

 that a ftone, which was not fufible under a heat of thirty- 

 eight degrees of Wedgewood's pyrometer, yields a glafs 

 that foftens at fourteen degrees ; and when this glafs is rc- 

 melted, and acquires a ftony texture by flow cooling, it 

 cannot be fufed again with a lefs degree of heat than thirty- 

 iire degrees. 



Pitch-Hone (fee PiTCH-Stone), though lofs vitreous in its 

 appearance than obfidian, yet, viewed by the geologift as it 

 exifts in nature, cannot be feparatcd from it, but mull be 

 clafTed as a different mode of the fame fubftance, or as 

 vitreous lava. The bafe of the Peak of Teneriffe, to the 

 plain of Ketama, is buried under fcorioe and heaps of pu- 

 mice reduced to powder. From thence to the fummit of the 

 mountain, or from fifteen hundred to nineteen hundred 

 toifes in height, the volcano exhibits only vitreous lavas, 

 compofed of obfidian and pitch-llone more or lefs porphy- 

 ritic : they are of blackiAi-brown, often varying to the 

 deepeft olive-green ; tliey contain large cryftals of felfpar. 

 The analogy of thefe decidedly volcanic fubitances with the 

 pitch-Hone porphyries of the valley of Turbach in Saxony is, 

 fays Humboldt, very remarkable ; but the latter contain 

 quartz, which is wanting in the modern lava. When the 

 lava changes from pitch-ftbne to obfidian, the colour i« 

 paler ; fometimes both varieties occur in the fame fragment. 

 Among the pitch-Rone and lava, near the fummit, were 

 found blocks of real greeniih clink-llone porphyry, fimilar 

 to the porphyry-flate of the mountain of Belin, in Bohemia. 

 Thefe faSs further prove the conneftion between rocks of 

 the trap formation and volcanic produfts. ( See Trap. ) 

 Obfidian and pitch-ftone arc found in Hungary, in Mexico, 

 and in Quito, at a great diftance from burning volcanoes. 

 Pitch-done cxiils abundantly in fome of the Scotch He- 

 brides, particularly in the ifle of Eigg. In South America, 

 obfidian is fcattered over the fields in angular pieces, and 

 fometimes forms ifolatcd rocks. The Mexicans dug ob- 

 fidian in mines, and mad<' knives, fword-blades, and razors 

 of this mineral. The Guanches in Teneriffe made ipear- 

 heads of obfidian ; it was alio employed by them, and by 

 the Mexicans, in the fabrication of mirrors and ornaments 

 for the women. Various volcajiic glaffes, differing in colour 

 and from obfidian, occur in fome volcanoes, particularly 

 that of the ifle of Bourbon. Thefe may, however, all be 

 clafTed with vitreous lavas, as it appears from the experi- 

 ments of M. Cordier, that the conllituent parts of all arc 

 the fame, being principally compofed of varying propor- 

 tions of augite and felfpar. 



Pumice (fee Pumice) is an abundant produft of vol- 

 canoes : it may be confidered as light fpongy lava, under 

 which term is comprifed a great variety of volcanic fub- 

 ftances, differing in porofity, in texture, and in colour. 

 The term pumice-Jlonc indicates a capillary or fibrous texture 

 of lava. It appears to be the produft of intcnfe heat, 

 operating cither on lava or obfidian ; the lighter coloured 

 pumices being formed of thofe volcanic rocks which abound 

 in felfpar, or rather it is the elements of thefe rocks in a ca- 

 pillary form. As fome obfidian fwclls greatly, and lofes its 

 colour by heat, it was inferred that all pumice has been 

 formed from this mineral ; but the conclufion is too general. 

 There are numerous inflances in wliich obfidian may be 

 traced pafGng into pumice ; but there are other inflances in 



which ilony lava, abounding in felfpar, may a]fo be traced 

 pafling into pumice-ftone. Some experiments made by 

 Humboldt prove that different obfidians fwell very un- 

 equally, when expofed to the moderate fire of a forge. 

 Thofe from the Peak of Teneriffe, and the black varieties 

 from Cotopaxi, increafed in bulk more than five times. 

 The red varieties from the Andes, on the contrary, were 

 not much tumefied by heat. We have already Hated in- 

 flances of currents of obfidian covered with pumice, and of 

 malfes of obfidian pafTing into pumice, fo as to leave no 

 doubt of the formation of pumice from obfidian. Nor are 

 there wanting inflances as decifive of lava pafling into pu- 

 mice. This cannot, on refleftion, appear furprifing, at 

 obfidian and lavas are effentially the fame fubftances in a 

 vitreous and flony form. Spallanzani defcribcs a lava witli 

 a bafe of felfpar, which is fpread over a part of Lipari, rifing 

 in rocks and craggs of enormous fize : it is of a grey colour. 

 On attentively examining this lava, the gradual tranfition 

 into pumice may be diftinftly perceived. It is not uncom- 

 mon to find maffes of this lava, which on one fide retain the 

 charafter of felfpar, and on the other are changed into white 

 pumice, exaftly refembling that of Campo Bianco in colour, 

 lightnefs, Itrufture, and other charafters. Some of the 

 white pumices of Campo Bianco are fo compaft, that the 

 fmallefl pore is not vifible to the eye ; but when vievred 

 through a lens with a ftrong light, they refemble an irre- 

 gular accumulation of flakes of ice : their compaftnefs, 

 however, does not prevent their fwimming on water. Other 

 pumices were full of pores and vacuities of a larger fize, and 

 their texture is formed by filaments arranged parallel to each 

 other, and of a filvery whitenefs : both thefe varieties may 

 fometimes be feen in the fame ftone ; hence we may infer 

 that the difference arifes from the aftion of elaflic fluids 

 producing different degrees of dilatation, when the mafs was 

 in a fluid ftate. There is a black pumice in Lipari, compofed 

 of parallel filaments, that all lie in one direftion, which is 

 that of the bed defcending from the mountain to the fea. 

 This, fays SpallanKani, may be confidered as a true current 

 of pumice. The black colour he fuppofed to proceed from 

 fome bituminous fubftance, as a ftrong fmell of bitumen is 

 emitted, when two pieces of this pumice are rubbed to- 

 gether. The black colour was entirely loft by expofure to 

 heat for fome time in the furnace, wliich reduced it to a 

 vitreous pallc. Humboldt conjeftnres that the dark cobur 

 of fome obfidians is caufed by a hydruiet of carbon. 



Nature, fays Humboldt, probably employs different 

 means to produce the fpongy and vitreous pumices of 

 Tenerifte, the pumices with parallel fibres from the Lipari 

 iflands, and the capillary vitrifications of the ifles of Bourbon, 

 which fometimes refemble a fpider's web. Thefe differ- 

 ences probably confift in the different degrees of heat, in 

 the different prcffure under which the fire afts, and in the 

 nature of the rocks ahercd by it. Above all, fays the fame 

 traveller, the prcffure which obfidians undergo in their 

 fufion, explains why thefe fubftances, with fome exceptions, 

 are never found whitened. Thofe pumices, which have the 

 appearance of having been formed at great depths, are 

 fibrous, and of a filky luflre. Blocks of this kind on the 

 Andes, of eight or ten toifes in length, have the fibres 

 exaftly parallel with each other, and perpendicular to the 

 direftion of the beds. Several volcanoes do not throw out 

 any pumice ; and thofe that do, ejeft them only by their 

 crater after the flowing of the lavas. 



Vulcanic Sand. — The white powders which have been 

 called aflies are generally thrown out the laft, and indicate 

 the end of the eruption ; tliey confid entirely of white pumice 

 ground to powder. The black powders iffuc the firft, and, 



being 



