VOLCANO. 



lava will depend on its fluidity, the quatitity thrown out, 

 and on the more or lefs rapid declivity ot the mountain. 



From various experiments made by Spallanzani and 

 others, on the melting of lavas, it appears that they are 

 fufceptible of different degrees of fluidity, according to the 

 degrees of temperature to which they are fubjefted. Thefe 

 gradations of fluidity, proportioned to the degree of heat, 

 take place in other ftones or fubftances fufible by fire, as 

 may be obferved in the flag from our furnaces. In fome 

 inllances, the lava appears to have the perfeft fluidity of 

 water. According to profeflbr Bottis, who was an eye- 

 witnefs in 1776, the lava fpoutcd from three fmall cones or 

 apertures on Vefuvius, precifely like water, forming three 

 beautiful fountains of fire, which defcribed curves of dif- 

 ferent dimenfions as they fell. He fays alfo, that he has 

 twice feen the inflamed matter break forth and difgorge in 

 the Atrio del Cavallo, at the foot of the volcanic cone of 

 Vefuvius. From its great fluidity, it refembled water 

 ifluing with violence from under the earth, and inundating 

 the adjacent country. The current of lava, which flowed 

 from this mountain in 1776, ftruck upon the lava of 1771, 

 and rebounded into the air, congealing in various figures, 

 terminating in thin fliarp points like needles. In the erup- 

 tion of 1754, the lava formed two branches, which flowed 

 thirty feet in forty-five feconds, or above half a mile an 

 hour ; and uniting lower down, proceeded at the rate of 

 thirty-three feet in fifty feconds. In 1765, the lava is 

 ftated to have flowed at the rate of a mile an hour. An- 

 other branch of the fame lava is faid by fir William Hamil- 

 ton to have had a velocity equal to that of the river Severn 

 at Briftol. In 1776, a torrent of lava from the fummit 

 of Vefuvius was obferved to flow a mile and a half in four- 

 teen minutes. When the declivity is very gentle, the mo- 

 tion is flow, if the current is not prefled forward by new 

 fupplies of melted matter. Notwithftanding the velocity of 

 torrents of lava, their tenacity is much greater than might 

 at firft have been expefted. Sir William Hamilton informs 

 us, that the lava of Vefuvius in 1765, which flowed a mile 

 an hour, almoft refift;ed any impreflion made on it with a 

 long pole ; and fome large ftones, thrown upon it with great 

 force, did not fink, but making only a flight impreflion, 

 fwam upon its furface. The tenacity and refiltance of lavas, 

 even when flowing, is, fays Spallanzani, an evident confe- 

 quence of the aftion of the cold atmofphere. The lofs of 

 heat fo occafioned is incomparably greater on the furface 

 than in the internal parts, in which the lava ft:ill retains a 

 confidenible degree of fluidity, as appears on breaking the 

 cruft. The different currents of lava from Etna have 

 flowed to the diftance of fifteen, twenty, and even thirty 

 miles from their fource ; and the current of lava, which 

 flowed during the volcanic eruption of 1783 in Iceland, ex- 

 tended nearly fixty miles in length. 



Neiu Volcanoes When a volcano breaks out in a new 



fituatioD, the phenomena are generally fomewhat different ; 

 but it may be proper to remark, that we have no inflances 

 of volcanoes breaking out on land, in countries that are not 

 or have not formerly been volcanic. New openings have 

 indeed been made, at the diftance of feveral m.iles from any 

 exifting volcano ; but they have taken place in a volcanic or 

 bafaltic foil. From prcfent appearances we are warranted 

 in the conclufion, that all volcanoes were originally fub- 

 marine. The moft remarkable inftance of the formation of 

 a range of volcanic mountains in a new fituation is that 

 recorded by Humboldt of the volcano of JoruUo, and 

 the adjoining hills, in the intendancy of Valladohd, or Me- 

 choacan, in New Spain, on the Z9th of September, i"59. 



A vaft plain extends from the hills of Agtiafarco nearly 

 2 



to the villages of Teipa and Petatlan. This plain is in fome 

 parts not more than Ivio thoufand fix hundred feet above the 

 level of the fea : it contains various conical hills of bafalt 

 and porphyry, crowned with evergreen oaks and palm-trees. 

 Till tlie middle of the eighteenth century, part of the plain 

 was cultivated with fugar-canes and indigo. It was bounded 

 by bafaltic mountains, the ftrufture of which indicated that, 

 at a very remote period, this country had feveral times been 

 convulfed by volcanoes. Thefe fields, watered by artificial 

 means, belonged to the plantation of San Pedro de Jorullo, 

 one of the largeft and richeft in the country. In the month 

 of June, 1 759, hollow fubterranean noifes of a moft alarm- 

 ing kind were accompanied by earthquakes, which fucceeded 

 each other for fixty days, to the great confternation of the 

 inhabitants. After the commencement of September, tran- 

 quillity appeared re-eftabliflied ; but on the nights of the 

 29th and 3Cth, the horrible fubterranean noifes were re- 

 newed. The affrighted inhabitants fled to the mountains of 

 Aguafarco. 



A tracl of ground, ten Enghfli miles in extent, rofe up in 

 the fliape of a bladder above the old level of the plain. 

 Near the edges it is only thirty-nine feet above the plain ; 

 but tqwards the centre, the convexity of the ground rifcs to 

 the height of five hundred and thirty-four feet above its 

 former level. This part of the ground is called Malpays. 

 Thofe who witnefled the fcene from the top of Aguafarco 

 aflert that flames were feen to iffue forth from an extent of 

 more than half a league, that fragments of burning rocks 

 were thrown to a prodigious height, and that the loftened 

 furface of the earth feemed to fwell like an agitated fea. 

 The rivers Cuitambo and San Pedro precipitated ihemfelves 

 into the burning abyfs, and appeared to invigorate the 

 flames. Torrents of mud and clay, enveloping balls of 

 bafalt in concentric layers, were thrown out. Thouiands 

 of fmall cones rofe up in the Malpays, from each of which 

 a thick vapour afcended. In the midft of thefe cones was 

 opened a large chafm, from which were thrown out fix large 

 mafles or mountains, from thirteen to tjxteen hundred feet 

 in height above the level of the plain : the moft elevated of 

 thefe is the volcano of Jorullo. Here we have a range of 

 volcanic hills formed in a few days, in the fame manner as 

 the Monte Nuovo near Naples, but of an extent and eleva- 

 tion exceeding that of the Malvern hills in Worcefterfliire, 

 or the Pentland hills near Edinburgh. 



The volcano of Jorullo is ftill continually burning, and 

 according to M. Humboldt's account, who vifited it in 

 1803, it has thrown up on tlie north fide an immenfe quan- 

 tity of fcoriie and bafaltic lavas, containing fragments of 

 prim.itive rocks. The firft great eruption continued to the 

 year 1760; m the following years, the explofions became 

 gradvially lefs frequent. The traveller is ftill fhewn where 

 the rivers Cuitambo and San Pedro difappeared on the 

 night of September 29, 1759. About one mile and a half 

 lower down now rife up two rivers, impregnated with 

 mineral matter, having a temperature of 126' Fahrenheit. 



There is one circumftance attending this great eruption, 

 which feems peculiar to the formation of new volcanoes or 

 volcanic cones. An immenfe rent is made in the furface, 

 through the whole of which the matter is ejedled, until the 

 chafm becomes choaked up in different parts, and the erup- 

 tions are confined to a few openings, round which the matter 

 is accumulated, forming a feries of craters or mountains, 

 ranged in one line. A fimilar range of volcanic cones was 

 formed on the fide of Etna, neareft Lingua Groffa, in the 

 year 1809. In this eruption, nine new boccas or craters 

 were formed in the fame fine, near to each other. The an- 

 cient volcanoes in Auvergnc, which are ranged in a line of 



fixty 



