VOLCANO. 



being driven with greater force, are carried to a greater 

 diftaiice from the mountain. Thefe powders aie called by 

 the Italians black and white rapiUi. 



VoLanlc fcoria or Jlags differ from pumice by their 

 greater denfity ; they are properly mafles of cellular lava, 

 and are more or lefs vitreous or ftony, according to the 

 degree of heat to which they have been fubjefted, and the 

 circumftances under which they have cooled. The upper 

 part of modern currents of lava, that have flowed in contaft 

 with the atmofphere, are generally compofed of fcorix. 

 The compofition of fcoriae is the fame as that of lava, and 

 varies with the different lavas from which it is formed. 



folcanic tufa appears to be formed of the loofe fand or 

 powders, together with the fmaller fragments thrown out 

 of volcanoes, which are fpread over the furface of the 

 ground, and afterwards become partly confolidated by 

 water and prefFure. In all fubmarine volcanoes, thefe pow- 

 ders muft be mixed with water as foon as they are dif- 

 charged from the mouth of the crater, and mull therefore 

 fall as a muddy fediment over the bed of the ocean, and 

 form ftrata of tufa of greater or lefs extent according to the 

 quantity of matter ejefted. The materials of which the 

 powders are formed, may alfo have been mixed with water 

 in the deep receffes of the volcano, and have been difcharged 

 in torrents of mud like thofe which ilfue from the American 

 volcanoes. In this way beds of tufa of great extent have pro- 

 bably been formed, and as they fometimes take the fame fhape 

 as the original inequalities of the ground, it has been fuppofed 

 that they have flowed as lavas. Spallanzani defcribes abed 

 of tufa in Lipari which covers the furface of the hills and 

 valleys nearly equally ; but it would be difficult to conceive 

 how a ftream of mud could afcend a hill, were it ever fo te- 

 nacious. If the matter were depofited from the turbid 

 waves of the ocean, we Ihould have no difSculty in account- 

 ing for its prefent appearance, and alfo for the flratifaftion 

 of tufa alternating with beds of lava. Volcanic tufa, in its 

 more indurated Hate, is ufed as building ftone ; foft or inco- 

 herent tufa has received the names of puzzolana, tarras, &c. 

 See Puzzolana. 



Volcanic tufa compofes the principal foil of many vol- 

 canic diftrifts. A great part of mount Etna and the 

 mountains on its fides are compofed of this tufa. 



Hills of tufa, according to fir G. Mackenzie, invariably 

 accompany lava in Iceland. Whole ranges of mountains are 

 formed of it, and wherever eruptions have occurred, thefe 

 hills of tufa may be feen. It clofely refembles the tufa of 

 Sicily and Italy. The tufa of Iceland often alternates with 

 fubmarine lava, and then it invariably includes maffes of 

 lava and flags, more or lefs rounded by the aftion of 

 water. The beds of tufa are fometimes not lefs than forty 

 feet in thicknefs. When tufa alternates with beds of 

 amygdaloid trap and greenftone, it includes raaffcs of thefe 

 fubftances. The fubmarine lavas which alternate with tufa, 

 are always above the beds of trap and greenftone alternating 

 with the fame fubilance. Sir G. Mackenzie conjeAures 

 that they are all the produfts of fubmarine volcanoes, but 

 that the beds of trap and greenftene were firft erupted at a 

 greater depth under the fea, and under a greater compref- 

 fivc force ; to which caufe the difference in their ftrufture 

 from that of the upper beds is to be attributed : hence the 

 lower beds, being more compreffed and compaft, have lofl 

 the appearance of the immediate aftion of fire which is fo 

 vifible in the cellular lava and flags nearer the furface. 

 Mountains of tufa, one thoufand feet in height, occur in 

 Iceland, and even whole mountain ranges are compofed of 

 the fame material ; in thefe there is no appearance of regu- 

 larity, but all the mafs is heaped up iii confufion; The pre- 



vailing colour of the pafte of this tufa is yellow ; and, in a 

 defcription given by Mr. Stephenfon of an eruption from 

 one of the Icelandic volcanoes, called the Kattlagian Jokul, 

 we have an inftance of its aftual formation. " The fand 

 which fell afterwards united, and covered the meadows with 

 a yellow-coloured cruft, quite compaft." 



The mud thrown out of the American volcanoes, when 

 indurated, may be claffed with tufa ; but belides the earthy 

 ingredients, it contains a portion of carbonaceous and faline 

 matter. To fome intermixture of this kind, the fertilizing 

 properties, afcribed to the fine fand or powder recently 

 ejefted from the volcano at St. Vincent's, may perhaps be 

 attributed. 



Puz'x.olana and terras are thofe foft tufas which are fer- 

 ruginous, and poflefs the property of confohdating under 

 water when mixed with lime as a cement. This property is 

 derived from the iron, and is common to many of the ar- 

 gillaceous Umeftones of England that abound in iron. 



From the experiments and obfervations of M. Cordier 

 before flated, it appears that the different earthy produfts of 

 volcanoes, whether as ftone in the form of compact, veficular, 

 or amygdaloidal lavas, or in a ftate of perfeft vitrification as 

 obfidian, or lefs perfeftly vitrified, as fcoriae, or in the earthy 

 form of wacke or volcanic tufa, or in beds of fand formed of 

 minute detached grains or particles, are all compofed princi- 

 pally of augite and felfpar in different proportions. 



This view of the fubjeft tends greatly to Amplify our 

 knowledge of volcanic produfts, as all the earthy maffes 

 and rocks ejefted from volcanoes, however differing in ftruc- 

 ture, denfity, and colour, ai'e to be regarded only as differ- 

 ent aggregations of the fame mineral fubftances, modified by 

 the various efFefts of heat and compreflion, and the operation 

 of thefe caufes to which they have been fubjefted fince their 

 firft eruption. 



Various rocks which have been claffed under the un- 

 fcientific denominations of fietz trap rocks and greenftone 

 are alfo compofed of the fame mineral fubftances aggre- 

 gated in a fimilar manner ; hence we may infer that they 

 have had a fimilar origin. Thefe rocks are very extenfively 

 fpread both in volcanic diftrifts, and in countries remote 

 from any aftive volcanoes ; they ferve as monuments to 

 elucidate the natural hiftory of the globe, and to mark the 

 boundaries of the ancient dominion of fire over the prefent 

 continents. 



Volcanoes in the Moon. Dr. Herfchel, now fo well 

 known and univerfally celebrated, on account of his va- 

 rious aftronomical obfervations, difcovered, on the fourth 

 of May, 1783, a burning volcano in the moon. This 

 difcovery confirms the conjeftures formed by M. jEpi- 

 nus, in 1778, and publifhed in a memoir printed at Ber- 

 lin in 1 78 1, concerning the volcanic origin of the ine- 

 qualities in the moon's furface. Similar ideas occurred to 

 profeffor Beccaria, of Turin, nearly at the fame time, and 

 alfo to profeffor Lichtenberg, of Gottingen. The nephew 

 of profeffor Beccaria difcovered. Oft. 11, 1772, alumin- 

 ous fpot on the moon, during its total eclipfe of that night, 

 in or near the place marked Copernicus ; and from this time 

 the profeffor mentioned this obfervation in his public lec- 

 tures, as an evidence that the round cavities on the furface of 

 the moon were fo many craters of diftinft volcanoes ; adding, 

 that he confidered thofe ftraight radiations, or bright paths, 

 which are obferved particularly on the place of the moon 

 marked Tycho, as fo many torrents of the lava, which 

 fpouted off in fome former conflagration of a volcano. 



The reader may fee this account, given by the profeffor 

 himfelf, in a letter concerning the luminous appearance ob- 

 ferved by don Ulloa on the moon, during the total eclipfe of 



the 



