D. Forbes — On Volcanos. 319 



loose materials whicb formed its walls to soften and cement them- 

 selves together on the inside into a sort of compact stony tube of 

 communication with the lower regions, much more solid and resistant 

 than the rest of the masSj of which, as before described,, tlie entire 

 cone had been built up. Once this is the case, the molten lava, 

 forced up by the gaseous pressure below, frequently ascends into the 

 crater itself, and overflowing its brim pours down the outside of the 

 cone, just like water when placed over too rapid a fire is seen to 

 boil over the edge of the pot in which it is heated. 



These occasional overflows of lava explain how. in the section of a 

 volcanic cone layers of more compact lava- are so frequently seen 

 alternating with those of the porous scoriaa and volcanic sand before 

 described. 



In more rare instances, as for example in the eruption of Mauna 

 Loa, in the Sandwich Islands, in February, 1859, the lava is ejected 

 in so wonderfully liquid a condition, and in such enormous volumes, 

 as to present the appearance of a red-hot fountain ; the jet of molten 

 lava thrown up from the crater on that occasion is described as about 

 250 feet in diameter, and as rising some 500 feet above the level of 

 the brim of the crater itself. 



Occasionally, during an eruption, the rim of the crater, unable to 

 support the weight of the molten lava which fills it, gives way at its 

 weakest point, the lava bursting out and carrying away one side of 

 the cone itself; at other times the lava, after having risen some height, 

 up the crater, finds out a point of weakness and breaks through, dis- 

 charging itself by a fissure some way up the side of the cone, as was 

 the case with the volcano of Sajama, in Bolivia, in 1859^ and with 

 Etna in 1865. 



In many eruptions the lava does not ascend at all into- the crater, 

 but breaks out at the very base of the cone, or even at some consider- 

 able distance from it through some subterranean passage ; this took 

 place in the eruption of Kilauea, in the Sandwich Islands, in June, 

 1840, when the lava first showed itself at the surface at Arare, some 

 six miles eastwards of the crater which supplied it. 



In fact, most volcanos will, upon examination, be found, at one 

 or other period of their history, to have presented examples of more 

 than one, if not of aU these different modes of discharging their 

 molten products. 



The eruption of Etna, in 1865, which I witnessed, did not proceed 

 from the summit or main crater, but broke out on the side of the 

 mountain, about 5,000 feet above the level of the sea. Along the 

 fissure or rent formed by the convulsion, no less than seven distinct 

 cones rose up at intervals, building themselves up very rapidly from 

 the enormous quantities of scorige which were thrown up from their 

 vents ; as they became larger, the bases of several of these cones ex- 

 tended until they united, and so formed a range of hills, the summits 

 of which, in but a few weeks, reached the height of several hundred 

 feet, and totally changed the character of scenery of this part of the 

 island. The four lowest cones were the most active, but from none 

 of their craters was there any overflow of lava, which, however 



