1859.] Sir C, Lyell on the Conical Form of Volcanoes. 125 



WEEKLY EVENING MEETING, 



Friday, April 15, 1859. 



Sir Roderick I. Murchison, D.C.L. F.R.S. Vice-President, 

 in the Chair 



Sir Charles Lyell, M.A. D.C.L. F.R.S. 



On the Consolidation of Lava on Steep Slopes, and on the Origin of 

 the Conical Form oj Volcanoes. 



During two recent excursions made in the autumns of 1857 and 1858 

 to Mount Etna, Sir C. Lyell had an opportunity of examining sections 

 of lava-currents of known date, which had descended steep slopes, and 

 had consolidated thereon in tabular and stony masses, the inclination 

 of which sometimes exceeded 30^. This fact has an important bear- 

 ing on the theory of " craters of elevation," it having been affirmed by 

 geologists of high authority, that when lavas congeal on a declivity 

 exceeding 5° or 6^, they never form continuous beds of compact 

 stone, but consist entirely of scoriaceous and fragmentary materials. 



The origin of such mountains as Etna and Vesuvius had of old 

 been referred to the cumulative effect of a long series of ordinary 

 eruptions, it being seen that reiterated showers of ashes and streams of 

 lava were often poured out from a permanent central vent. This 

 opinion was advocated by Mr. Scrope in his work on volcanoes in 1825, 

 and by Sir C. Lyell in his Principles of Geology , after his exploration 

 of Vesuvius and Etna in 1828 ; both authors considering the injection 

 from below of melted matter, in the shape of dykes, as part of the 

 cone-making process. 



But in place of this simple explanation of the phenomena. Von 

 Buch substituted the following hypothesis : that a vast thickness of 

 horizontal or nearly horizontal sheets of lava and scoriae, having been 

 first deposited, an expansive force operating from below, exerts a 

 pressure both upwards and outwards, from a central axis towards all 

 points of the compass, so as suddenly to uplift the whole stratified 

 mass, making it assume a conical form ; giving rise at the same 

 time, in many cases, to a wide and deep circular opening at the top of 

 the cone, an opening called a '* crater of elevation." 



In all great volcanoes of which sections can be obtained, there are 

 some layers of compact stone, inclined at angles of 10°, 20^, and 

 sometimes much higher angles, and these beds are often among the 

 uppermost, or last formed of the whole series. Hence it was logically 



