VOLCANO. 



In order to fonn a moreexaft idea of the external ftrufture 

 of volcanoes, it is important to compare their perpendicular 

 lieiglit with their circumference ; but this can only be done 

 with ifolated mountains placed on a plain which is nearly 

 on a level with the fea. The height of the Peak of Tene- 

 rifFe is one twenty-eighth of the circumference of its bafe ; 

 that of Vefuvius, according to Von Buch, is a thirty-third ; 

 and of Etna, a thirty -fourth. Ifolated volcanoes, in the 

 mod diftant regions, are very analogous in their external 

 "ftrufture. All have elevated plains, in the middle of which 

 rifes a cone perfectly circular. The greater the quantity of 

 matter that has ifTued from the crater of a volcano, the more 

 elevated is its cone of a(hes, in proportion to the perpen- 

 dicular height of the mountain. Nothing is more ftriking 

 than the difference in this refpeft, fays Humboldt, between 

 Vefuvius, the Peak of TenerifFe, and Pichinca. The cone 

 of Cotopaxi, the form of which is the moft regular and ele- 

 gant of any hitherto known, is 540 toifes in height, but it 

 is impoffible to decide whether the whole of this mafs is 

 covered with afbes. 



Cone covered with Aflies. 



The latter column fhews the proportion of the cone to 

 the total height of the mountain. 



In moft volcanic mountains, the cone, or fugar-loaf, as it 

 has been not unaptly called, prefervcs its conic figure to 

 the very fummit ; the whole of the declivity is inclined the 

 fame number of degrees, and is uniformly covered with 

 layers of voltanic fand or powder. When we reach the top, 

 nothing obftrufts the view of the bottom of the crater. 

 The Peak of TenerifFe and Cotopaxi, on the contrary, liavc 

 a different conftruiElion. Their fummits have a circular 

 ■wail, furrounding the brink of the crater, which appears at 

 a diitance like a fmall cylinder placed on a truncated cone. 

 According to Humboldt, this pecuhar conllruftion of 

 Cotopaxi, is vifible to the naked eye at the diftance of 

 nearly three leagues. No perfon has reached the crater of 

 this volcano. On the Peak of TenerifFe, the wall that fur- 

 rounds the crater is fo high, that it would be impoffible to 

 enter, if there were not a breach which feems to have been 

 made by the flowing of an ancient current of lava. 



The (hape of volcanic craters is generally that of a funnel, 

 either circular or elliptical, the fides fhelving down to the 

 bottom, which is a plain of greater or lefs extent, having 

 apertures or fifFures, through which fmoke and heated 

 vapour are exhaled. At the bottom of many volcanic 

 craters are one or more fmall cones, which during erup- 

 tions enlarge, and fometimes fill up the crater, and rife 

 above its brim. The prefent cone of Vefuvius is fuppofed 

 to have been railed within a crater of much larger fize, of 

 which mount Somma forms part of the remaining wall. 

 ( See Vesuviu.s. ) The fize of the crater does not depend 

 on the height and mafs of the mountain, of which it 

 forms the principal vent. Vefuvius, which is but a fmall 

 hill compared with the Peak of Teneriffe, has a crater 

 with a diameter five times larger than that of the latter 

 mountain ; and the prefent crater of Vulcano equals or 

 exceeds that of Vefuvius, though the height of the cone is 

 not more than 1500 feet above the level of the fea. Wiien 

 we reileft, fays Humboldt, that very lofty volcanoes throw 

 out lefs matter by their fummits than by lateral openings, we 

 ihould be led to conclude that the lower volcanoes are, their 

 force and aftivity being the fame, the more confiderable 



ought to be the fize of their craters. There are immenfe 

 volcanoes in the Andes, which have but verj- fmall open- 

 ings, and we might eflablifh it as a geological principle, that 

 the moft lofty volcanoes have craters of fmall extent at 

 their fummits, if the Cordilleras did not offer many inftances 

 to the contrary-. The great volcanoes of Cotopaxi and 

 Rucupichinca have craters, which, according to the admea- 

 furement of this indefatigable traveller, exceed half and 

 three-quarters of a mile in diameter. 



In a volcano like Vefuvius, the attivity of which is prin- 

 cipally direded towards the fummit, the depth of the crater 

 varies before and after every eruption ; but at the Peak of 

 Teneriffe, the depth of the crater appears to have been fta- 

 tionai-y for a long time. In 1 7 1 5, it was eftimated by Mr. 

 Eden at one hundred and fifteen feet; in 1805, by M. 

 Cordier, at one hundred and ten feet ; and fubfequently, by 

 Humboldt, it was conjedlured to have rather lefs depth. 

 The infide of the crater indicates a volcano, which for a long 

 period has emitted no fire at the fummit. From the lapfe 

 of time, and the aftioii of vapours, the infide walls have fallen 

 in, and have covered tlie bafin with great blocks of lava. 



For an account of the cone and crater of mount iEtna, 

 fee ^TNA. 



Among the various changes that have taken place in 

 this volcano, it is highly probable that the partition 

 between the upper and lower crater may have been fre- 

 quently removed. Lieut. -general Cockburn, who vifited 

 Etna in 18 10, defcribcs only one crater, though he afcended 

 the highefl pinnacle. This crater, he eftimates at nearly 

 two miles in circumference. At that time the bottom of 

 the crater, which he diftinftly faw, was not flat ; it con- 

 tained fevcral minor mountains and their craters, fome 

 fmoking like tlic moft violent glafs-furnace, or fteam- 

 engine. Cockburn's Travels in Sicily, vol. i. p. 137. 



The whole cone of a volcano is fometimes fwallowed up 

 during an eruption, leaving a circular crater of a larger 

 diameter and at a much lower level ; which, when the 

 volcanic fire becomes extindl, or remains dormant for 

 ages, may form a lake. The celebrated lake of Avernus, 

 near Naples, and the neighbouring lake Agano, are the 

 craters of extinft volcanoes, the cones of which have pro- 

 bably been buried after a great eruption, or by an earth- 

 quake. Numerous circular lakes exifl in volcanic countries 

 which have had the Came origin. Nor need we be furprifed 

 at the difappearance of a volcanic cone, however large, as ic 

 muft ftand and have its foundation on the brink of a much 

 larger abyfs, from which it has been thrown out, as we (hall 

 have occafion to remark in defcribing the formation of fome 

 of thefe cones, which have taken place in modern times. 



The crater of a volcano can only be approached when the 

 fire is in a dormant or nearly quiefcent (late ; but as the in- 

 tervals between volcanic eruptions fometimes laft for many 

 years, and even centuries, opportunities are offered for ex- 

 ploring their ftrufture. The floor of the crater appears in 

 many inftances to be only a thin congealed cruft, and returns 

 a hollow found when ftruck upon with a ftone or any hard 

 fubftance. This is the cafe at the Solfaterra, which ap- 

 pears to be the floor of an extinft crater. See Solfa- 

 terra. 



When M. de Luc walked over the bottom of the crater 

 of Vulcano in 1757, it returned a hollow found. The 

 largeft diameter of the crater was then above three-quarters 

 of a mile, and the depth nearly a thoufand feet. In 178 1 

 it was vifited by M. Dolomieu, who found it impoflible to 

 enter the crater ; its depth he eftimated at half a mile from 

 the brink, and the bottom not more tlian two hundred and 

 fifty feet in diameter. He threw in fome large ftones from 



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