188 CHANGING POSITION OF VOLCANIC VENTS. 



It is an almost universal experience, after the crater of a volcano 

 has been thus formed, and the lava raised and poured out, that con- 

 siderable quantities of heated water rise through the molten mass in 

 the volcanic throat, and burst in great bubbles so as once more to 

 throw up cinders, which fall around the throat and begin to build up 

 a new cone within the crater. The views of Vesuvius published by 

 Sir William Hamilton are particularly instructive in this connection. 

 In 1756 this mountain possessed no fewer than three cones and 

 craters, rising successively one within another, the outermost being 

 girdled by Monte Somma. Then the innermost cone became obliter- 

 ated, and finally, in 1767, but one cone existed within the crater, 

 which in due time became filled up, so as to form a convex platform ; 

 until a new eruption, in 1822, blew out the entire centre of the 

 mountain, and then once more small craters began to appear as the 

 crater, became filled up again. More than once, two or more minor 

 cones have existed side by side within the crater of Vesuvius, as 

 though a fissure had formed across its floor, and permitted the 

 escape of the explosive forces along a line. 



Volcanoes without Cones. It is not always, however, that cinder 

 cones are produced. In the summit crater of Mauna Loa, which is 

 termed Mokua-Weo-Weo, there were at the bottom of the pit that 

 serves as crater two cinder cones, one of them rising 200 feet in 

 height when examined by Professor Dana. Eut the Sandwich Island 

 volcanoes are remarkable for the absence of cones, in place of which 

 there are deep pits on the side of the mountain, and the so-called 

 cones are patches of lava which have been ejected on the flanks. 

 Kilauea, which is sixteen miles from the summit of the mountain, 

 has no cone, and the crater is a pit 7^ miles in circumference. 



Cones arranged in Lines. Any one who carefully examines a 

 volcano, or still better, a volcanic district, will observe that the posi- 

 tion of the eruptive vent undergoes some change. Thus, in the island 

 of Vulcano, there is the ancient crater which forms the S.E. of the 

 island, with three other craters successively superimposed, before we 

 reach the small mass on the north termed Vulcanello, which also has 

 a succession of craters in a line. But it is more instructive to observe 

 the manner in which cones succeed each other on a grand scale in 

 such a chain as the Andes, or the Kurile Islands ; for we may reason- 

 ably infer that the cause, whatever it was, whicli determined the 

 shifting of the point of eruption along such a line as that of Vulcano, 

 is closely analogous to the cause which determines the intermittent 

 outbursts of eruptions along successive portions of a volcanic chain. 



We have only to observe what has happened in the history of Etna 

 to learn the secret of the linear extension of volcanic vents. After 

 the central cone has become sufficiently massive and consolidated to 

 oppose a resistance which the explosive forces below cannot easily 

 overcome, they occasionally find an outlet by producing rents on the 

 mountain-side. Rarely these rents radiate, but more frequently they 

 are limited to one side of the mountain, and sometimes occur in 

 parallel bands. Among numerous illustrations given by Ferrara, we 



