VOLCANO. 



rocks of ererj age and country, that have flowed as lavas, 

 or been ejefted during fiery eruptions, are compofed of the 

 fame mineral fubftances, and are different in their compofi- 

 tion and internal ftrufture from rocks which form the regu- 

 lar ftrata of the globe. 



The external ftrufture of lava is much diverfified, owing, 

 in all probabihty, to the different circumflances under 

 which it has cooled. Some lava is porous, fome contains 

 large cavities or is veficular, whilfl other lavas are appa- 

 rently compaft, and afTume a prifmatic form. According 

 to the obfervations of fir G. S. Mackenzie in Iceland, there 

 are beds of lava of great extent, which appear never to have 

 flowed in currents, but to have been completely fufed in the 

 fituations where they occur. This lava was columnar in 

 many places, the columns varying in fize from a few inches 

 to feveral feet in diameter. The furface of the lava was 

 heaved up into large bhfters and bubbles, fome of which 

 were round, and from a few feet to forty or fifty in dia- 

 meter, others were long, and fome were waved. A great 

 many of the bubbles had burft, and difplayed caverns of 

 confiderable depth. On this account fir G. Mackenzie 

 denominates it cavernous lava. Currents of lava, which had 

 flowed from volcanoes, covered the cavernous lava in many 

 parts, but prefented very diftinft charafters. In the com- 

 mon ftreams of lava, no defined approach to a columnar 

 form was obferved ; but nothing was more common than 

 the columnar ftrufture in the cavernous lava. In fome parts 

 of Iceland were feen beds of amygdaloid, from ten to forty 

 feet in thicknefs, alternating with tufa. The upper part 

 of thefe beds did not indicate the adlion of fire, but the 

 under part of each was a complete volcanic flag. From the 

 fituation of thefe beds, and other circumflances, it was in- 

 ferred that they were lavas which had flowed under the fea. 

 Some of the beds were very compaft in the upper part. 

 Another feries of beds occur near Krifuvick, which was 

 flaggy at the bottom, but fo compaft above as to refemble 

 porphyry flate. Beds of very compaft bafalt, with the 

 under furface flaggy, were alfo obferved ; and an extenfive 

 and beautiful range of lofty columns at Stappen prefent the 

 fame appearance, and have flaggy maffes included in them. 

 Sir George Mackenzie explains thefe appearances, by fup- 

 pofing the lava to have originally flowed over a cold wet 

 furface at the bottom of the fea. An abundance of fleam 

 would conflantly be produced from the upper furface, 

 which would feparate the hot lava from the water, in the 

 fame manner as a drop of water is kept detached from a 

 plate of red-hot iron. Thus, ho water could enter the fub- 

 Itance of the lava from above, but the moifture below would 

 operate very differently. From its converfion into fleam, 

 and the tendency to afcend, it would penetrate the fluid 

 lava, and produce the porofity obferved in the above rocks, 

 and render the lava more or lefs veficular, according to its 

 degree of fluidity. When the lava is very hot and liquid, 

 the fleam will have lefs difficulty in penetrating it. In fome 

 inflances it may allow the whole of the moiflure to efcape 

 through it in the form of elatlic vapour, fo that the lava 

 may become fohd. According as the lava is more or lefs 

 vifcid, the fleam may be more or lefs confined, making the 

 Hone porous or veficular ; and, lailly, tlie lava may be fo 

 tough, that the exertions of the elatlic vapour may be con- 

 fined to the lower furface of the beds. In the firfl cafe, a 

 mafs of compaft flone would be formed, having no appear- 

 ance of the aftion of fire. In the fecond cafe, the lava 

 would form an amygdaloidal or veficular mafs. In the lafl 

 cafe would refult a mafs entirely compaft, except in the 

 under-furface. (Travels in Iceland, by fir G. S. Mac- 

 ;kenzie. ) In the formation of volcanic rocks, which have 



J 



flowed as lava under the fea, very different refulta would 

 take place from the formation of fimilar rocks on land, owing 

 to the great difference which the fuperincumbent preffure of 

 a deep volume of water would occafion ; and as moft of the 

 ancient currents of lava have in all probability been ori- 

 ginally fubmarine, we may expeft them to vary in flrufture 

 from the lavas of more recent eruptions. In the Tranfac- 

 tions of the Royal Society of Edinburgh are feveral valuable 

 papers of fir James Hall, detaihng a feries of the mofl in- 

 terefling and inflruftive experiments on the effefts of heat 

 modified by comprelfion. Thefe experiments merit the 

 profound attention of every one who would endeavour to 

 form a jufl and comprehenfive view of the agency of fubter- 

 ranean fire on the different rocks which form the crufl of 

 the globe. For the refult of fome of thefe experiments, we 

 refer to Systems of Geology; but we particularly recom- 

 mend our readers to perufe the original papers, which are 

 well illuftrated by a feries of plates. 



The minerals which line or fill the cavities of veficular 

 lava are principally varieties of zeolites, chalcedonies, and 

 calcareous fpar. Quartz cryflals abound in fome of the 

 veficular lavas of Lipari. All thefe minerals are fuppofcd, 

 with much probability, to be of poflerior formation to the 

 lava itfelf, and to derive their origin from the infiltration of 

 water, holding the conflituent parts in folution or fufpen- 

 fion. Spallanzani conjeftures that the particles are fepa- 

 rated from the lava itfelf, by the decompofing effefts of 

 fulphureous acid. 



Lava is fubjeft to decompofition from atmofpheric agency, 

 according as it is more or lefs vitreous. Some lavas are 

 known to have refilled all tendency to decompofe for many 

 centuries ; other lavas decompofe rapidly, and form a pro- 

 duftive foil. Particular vegetables poffefs the property of 

 reducing lava to vegetable mould with great rapidity. The 

 Indian fig, or, as it is commonly called, the prickly pear, 

 has this property in a remarkable degree. According to 

 the account of it given by General Cockburn, in his " Tra- 

 vels through Sicily," this plant pulverifes the hardefl rocks, 

 and forms the mofl luxuriant foil. The inhabitants bring a 

 little earth to any crevice of lava, and plant a prickly pear- 

 tree in it, which fpreads and fplits the rocks in about feven 

 years. A thick plantation is thus formed, and a very little 

 earth being added, in about ten years more the rock is pul- 

 verifed for fome inches deep. Vol. ii. p. 163. 



Objidian or black volcanic glafs appears to be a vitreous 

 modification of ftony lava, produced by its fudden re- 

 frigeration. According to the obfervations of Cordier 

 before flated, it may confifl either of felfpar or angite, as 

 forming the principal part of the bafe. The volcanic origin 

 of this mineral has been denied by fome geologifls without 

 any apparent reafon, except an attachment to theory ; for 

 this fubflance may be traced flowing from the craters of 

 volcanoes, and palfing into compaft black lava or bafalt, 

 and alfo into white fpongy pumice. Sir James Hall and 

 Dr. Home vifited a mountain in Lipari, that had efcaped 

 the attention of Dolomieu. From feveral openings in this 

 mountain a flream of obfidian and pumice miglit be traced : 

 they gradually paffed into each other. The pumice liad 

 evidently flowed with the obfidian, as it formed the upper 

 furface of the flream. The greatefl breadth of the flream 

 was about two miles and a half, and its length three miles. 

 It feemed to have been produced by the lafl effort of the 

 volcano. Sir G. Mackenzie difcovered a flream of obfidian 

 in Iceland, fiUing up a valley to the depth of thirty feet, 

 and vifible for more than two miles ii; extent. The furface 

 was in many parts covered with pumice. Obfidian is found 

 flraaming from the crater of Vulcano : it exifls in abun- 

 dance 



