J. W. Judd—On Volcanos. 355 



character as to blow into the air all the central portion of Etna, so as 

 to leave a crater of many miles in diameter, the result would be not very 

 different from the vast lake surrounded by a rim of comparatively 

 small elevation, which we witness in Bracciano and Bolsena. But 

 here we are met by the fact that, in Italy, at least within the historic 

 period, no such mountain as Etna has ever been so destroyed by a 

 volcanic outburst as to leave only a basal wreck consisting of a wide 

 and low crater-ring. 



Etna is an admirable type of a well-built volcano. As shown in 

 the splendid section of the Val del Bove, lava-streams, dykes, and 

 agglomerates are combined together into a framework of the 

 most solid character. As the structure has risen in height, the 

 weakest portions of its flanks have successively yielded to the vast 

 expansive forces below, and fissures being produced, these weakest 

 parts have been successively repaired and strengthened, first by the 

 injection and consolidation of lava in the fissures, and secondly by 

 the piling up of materials above them. Thus the grand cone has 

 grown, by the alternate strengthening of its flanks through lateral 

 outbursts, and the renewal of ejections from its axial crater, as the 

 vast chimney became sufficiently strong to sustain the pressure 

 necessary to raise the materials to the lofty summit of the 

 mountain. That this has really been the process of growth in Etna, 

 no one who studies its enormous bulk, its numerous parasitical 

 cones, and its clear sections, can for one moment doubt. 



But as we have already pointed out, the wide and little elevated 

 crater-rings of Albano, Bracciano and Bolsena present a totally dif- 

 ferent kind of architecture to the solid structure of Etna. They are 

 in fact almost wholly built up of loose tuffs ; masses of solid lava, 

 whether in currents or dykes, being few, and forming but a very 

 small proportion of their bulk. 



The action of expansive forces within cones almost wholly com- 

 posed of such loose materials would necessarily be very different 

 from that which we have seen takes place in Etna. Lateral eruptions 

 would become almost impossible, for as soon as any part of the flanks 

 of the mountain began to yield to the rending force, the loose materials 

 at the sides of the fissure would close in and fill the crack as rapidly 

 as it was formed. That this is no hypothetical explanation of what 

 takes place in such tuff cones is shown by the numerous beautiful 

 pseudo-dykes, filled with fragmentary materials, which occur in 

 the tuff-cones of the Campi Phlegrasi, and the almost total absence 

 in these cones of dykes of solid lava. 



The expansive force of the vapour, gradually separated from the 

 incandescent masses of lava below the mountain, being thus unable to 

 open any safety-valve by producing a lateral eruption, would at last 

 attain such tension as to enable it to dissipate the whole structure of 

 the cone itself, composed as it is of loose and uncompacted materials. 

 These by repeated ejection would be reduced to fine fragments, which 

 would be deposited as tuff and ash over enormous areas all around 

 the vents. The craters of Albano, Bracciano, and Bolsena are in fact 

 surrounded by such deposits, which extend over a wide district around 

 them. 



