
_ Parr I. Seer. i. § 3.) “ELEVATION CRATERS.” 241 
ground round the vent from which the materials are finally expelled. 
_ A portion of the earth’s crust (represented in Fig. 46 as composed of 
_ stratified deposits, a b g h) was believed to have been pushed up like 
a huge blister, by forces acting from below (at ¢) until the summit 
of the dome gave way and volcanic materials were emitted. At 
first these might only partially fill the cavity (as at f), but subsequent 
eruptions, if sufficiently copious, would cover over the truncated edges 
of the pre-volcanic rocks (as at g and h), and would be liable to further 
upheaval by a renewal of the original upward swelling of the site. 
my) ca (- Cae 
(6 LOR 
eS 
“Gla ve 
vy KSsec> 
es Ee 

Sod by oe 
Fic. 46.—SecTIon ILLUsTRATIVE OF THE ELEVATION-CRATER ‘THEORY. 
It was a matter of prime importance in‘the interpretation of 
voleanic action to have this question settled. To Poulett Scrope, 
Lyell, and Constant Prevost belongs the merit of disproving the 
_ Crater-elevation theory. Prevost pointed out that there was no more 
reason why lava should not consolidate on steep slopes than that 
tears or drops of wax should not do so. Scrope also showed con- 
clusively that the steep slope of the lava-beds of a volcanic cone was 
original.” Lyell, in successive editions of his works, and subsequently 
by an examination of the Canary Islands with Hartung, brought 
forward cogent arguments against the Elevation-crater theory? A 
- comparison of Fig. 46 with Fig. 47 will show at a glance the difference 
ary 
-. p. 50 
3 
between this theory and the views of volcanic structure now 
universally accepted. The steep declivities on which lava can 
actually consolidate have been referred to on p. 228. 
The cone grows by additions made to its surface during successive 
eruptions. Its angle of slope depends mainly upon the angle of 
repose of the erupted materials, but is apt to be modified by the 
effect of rain and torrents, in sweeping down the loose detritus and 
excavating ravines on the sides of the cone.* 
1 Comptes Rendus, i. (1835) 460; xli. (1855) p. 919. Gol. Soc. France: Mémoires, 
ii, p. 105, and Bull. xiv. 217. Société Philom. Paris, Proc. Verb. 1843, p. 13. 
2 Considerations on Volcanoes, 1825. Quart. Journ. Geol. Sce. xii. p. 326. 
3 Phil. Trans. 1858, p. 703. 
* On the slopes of volcanic cones, see J. Milne, Gecl. Mag. 1878, p. 339; 1879, 
6. 
R 
