284 Notice of Active and Extinct Volcanos. 



been forced up through the midst of the older basalts, and 2dly, 

 into the products of the volcanic action to which this central mass 

 furnished an appropriate vent. 



"The latter are very various in their nature and character: 

 we may distinguish, first, the lavas, which have sometimes a 

 stony, and sometimes a vitreous aspect; and secondly, the loose 

 ejected masses, such as pumice, obsidian, and lapilli. 



" Of the lavas, such as have a stony aspect, appear to be confi- 

 ned to a comparatively low elevation, and to have proceeded ex- 

 clusively from the flanks of the volcano — whilst the vitreous are 

 found only near the summit, the lowest point at which they occur 

 being- eight thousand nine hundred feet above the level of the 

 sea." 



There is however one vitreous current near the top of the 

 mountain ; and this region is covered with obsidian, and with 

 showers of pumice, but they do not reach the lower parts of 

 the mountain, which are mostly covered by rapilli, resembling 

 lithoid lava, and not mixed with pumice or obsidian. 



" This latter distribution, says Humbolt, seems to confirm the 

 observation made a long time ago at Vesuvius, that the white 

 ashes are thrown out last, and indicate that the eruption is at an 

 end. In proportion as the elasticity of the vapours diminish- 

 es, the matter is thrown to a less distance ; and the black ra- 

 pilli, which issue the first, when the lava has ceased flowing, 

 must necessarily reach father than the white rapilli. The last 

 appear to have undergone the action of a more intense fire. 



" The size of the crater that exists in the summit of the Peak 

 is diminutive compared with that of Etna or of Vesuvius, being 

 only three hundred feet in its greatest, and two hundred in its les- 

 ser diameter, whilst its depth does not exceed one hundred feet. 



" Indeed it may be remarked in general, although the rule is 

 liable to exceptions, that the dimensions of a crater are in an in- 

 verse ratio to the elevation of the mountain ; for in proportion 

 to the height which the ejected masses must attain before they 

 reach the orifice, will be the resistance to be overcome in for- 

 cing a passage by this channel, so that in a mountain like the 

 Peak of Teneriffe, the force applied will in most instances be 

 instrumental in creating apertures in the flanks of the mountain, 

 rather than in enlarging the cavity on its summit. 



" The existence nevertheless of this chimney preserves the 

 island in Von Buch's opinion from those destructive eruptions 

 which convulse some of those adjoining it, since elastic vapours, 

 the immediate and necessary concomitants, of volcanic action, 

 thus find a readier vent, and confine their violence to the imme- 

 diate precincts of the volcano. 



