24 © DYNAMICAL GEOLOGY. [Boox: 
7 ee 

tinctly crystalline structure inside (ante, p. 105). Dykes of this kind ~ 
with a vitreous crust on their sides may be seen on the crater-wall of 
Somma and not uncommonly among basalt dykes in Iceland and 
Seotland. In other cases the lava had probably already acquired a 
lithoid character while still rising in the fissure, and in this condition 
was able to push aside and even contort the strata of tuff through 



_——- > 
“SS 
= TSM 
= =| eae ee 
pes BM. 
ST RSRRE SS (= 
‘ 
es oe 
Ne <r 
m9 






Sailitz it Wes 
Wy KE aN 3 Btn YASIR ° 
SS a So 
ES Ai S 



i ny !°_ am 
hone EOT UN y LEY = 
ap ae Ai A 
a F Ss Bye “ 
=> 
Fic. 36.—DyYKE CONTORTING BEDS oF TuFF. CRATER OF VESUVIUS (ABICH). 
which it made its way (Fig. 86). There can be little doubt that in 
ae ee the architecture of a volcano dykes must 
ibaciives Peete =< act the part of huge beams and girders 
(Fig. 87), binding the loose tuffs and 
intercalated lavas together and strengthen- 
ing the cone against the effects of subse-— 
: quent convulsions. . 
Fic. 37.—Sroti0n or DYKES OF From this point of view an explana-— 
ee ee tion suggests itself of the observed alter- 
nations in the character of a volcano’s erup- 
tions. These alternations may depend in great measure upon the 
relation between the height of the cone on the one hand and the 
strength of its sides on the other. When the sides have been well 
braced together by interlacing dykes, and further thickened by the 
spread of volcanic materials all over their slopes, they may resist the 
effects of explosion and of the pressure of the ascending lava column. 
In this case the volcano may find relief only from its summit, and 
if the lava flows forth, it will do so from the top of the cone. As the 
cone increases in elevation, however, the pressure from within upon 
its sides augments. Eventually egress is once more established on 
the flanks by means of fissures, and a new series of lava-streams 
is poured out over the lower slopes. 
Though lava very commonly issues from the lateral fissures on a 
voleanic cone, it may sometimes approach the surface in them with- 
out actually flowing out. The great fissure on Etna in 1669, for 
example, was visible even from a distance by the long line of vivid 
light which rose from the incandescent lava within. Again, it. 
frequently happens that minor volcanic cones are thrown up on the 


