HARDWICKE'S SCIENCE-GOSSIP. 



221 



ments, to the height of many feet. These "masses 

 appear black by day, but red-hot by night ; they may 

 cool or not, before falling ; if the latter, when they 

 strike the ground, they adapt themselves to the irregu- 

 larities of the surface and form as it were a cast 

 thereof. This condition is much exaggerated at the first 

 outbreak of an eruption ; the vast column of fragments 

 often reach an altitude of two and three thousand 

 feet. There the pieces ascending meet those descend- 

 ing, and so there is a continual grinding going on 

 between them ; the fine dust is taken by the wind 

 and transported often many miles, forming the so- 

 called clouds of volcanic ash. The larger fragments 

 (or lapilli, as they are named) may again fall back 



and 45 , we find the strata composing the cone (D, E). 

 This arrangement is often called periclinal. The 

 funnel, or chimney, which has been mentioned as 

 occupying the centre, has the form of an inverted 

 cone, the inclination of its sides and its diameter 

 necessarily being proportional to the volume and 

 force of the escape of vapour, and also to the nature, 

 form, and size of the surrounding fragments, forming 

 the growing cone, which have already been ejected. 

 The upper, or basin part, is technically called the 

 crater. The vapour only may have made its ap- 

 pearance at the surface, and in fact may have parted 

 company with the lava at very considerable depths. 

 Or the latter may have been forced up almost 



r 



TO 



jc 11 ii U I'--- ( J if i'TtoV. ii_Jj-1L a' if V 

 --r~ =-i jX'TTiTi'I ii li it i! u-i'i—r- 



Fig. 136. — Diagrammatic bird's-eye view of a volcanic cone. The upper part is supposed to be removed by a horizontal section and 

 one half of the remaining base by another longitudinal one. A, vent ; B, chimney ; C, basement rock, compressed downwards 

 at C and upwards at C" ; D, ash-beds ; E, lava streams, one of which E' is seen to have run down the slopes G of the cone, 

 and spread over the plain F. 



into the opening or around its edge, thus building up 

 an annular bank. This is really the foundation of 

 the cone. 



If we speculate for a moment on the formation of 

 such a heap, we shall see that the first strata deposited 

 will be horizontal, but somewhat thicker towards the 

 axis of explosions. .See D, fig. 136. This, however, as 

 the action continues, will begin to arrange itself in a 

 direction slanting away from the axis, until the beds 

 reach the maximum angle of repose of the rock frag- 

 ments in question as the beds (DD) in fig. 136. Thus 

 we have constructed a conical mass in the centre of 

 which is the volcanic chimney (B) and dipping away 

 on all sides at angles varying generally between 20° 



simultaneously with the vapour, and poured out over 

 the edge of the primitive cone. This, however, is 

 not the general rule, for an escape of much gaseous 

 material nearly always precedes for a variable period 

 the appearance of the lava. In fact, when a volcanic 

 outburst has forced a convenient passage for the 

 vapour, the exit of liquid rock seems of secondary 

 importance, for generally the terrific explosions, earth- 

 quakes, and subterranean thunder that accompanies 

 the first stage of eruption is more or less absent, or at 

 least much diminished during the welling up of the 

 fluid rock. If, as in the latter case, a cone of some 

 considerable size has been formed, the lava will rise 

 and occupy the whole of the crater cavity. Two 



