1. 



^ 



^ 



** 



*ti 





rk 



: ^ei 



% 



of 



%i 



Uu 



His 





>H 



een 



o 



let. 



? 



»• la 



k 



is no 



ich the 

 >e lavas 

 P slope 



ilsaw 



? which 



u cone. 



le tk 



e edges 



lent of 

 matter 



t there 



and I 



of 



ence 



off, one 

 ■ traces 



>n, and 

 layers. 



Tiout 

 of 



% 



rers 



•ed-hot 



mtaiB- 

 • stony 



i 



W 



$&• 



Ch. XXV.] 



TO MONTE SOMMA OR TO VESUVIUS. 



081) 



I measured carefully the dimensions of one of the numerous 

 streams of 1857, and found it to be 50 feet wide, and inclined, 

 like the surface of the cone on which it rested, at angles 



Fig. 69 



Structure of successive juxtaposed modern lava-streams 



om 



Its average thickness was 



10 feet. Having 



Mount 



consolidated on still steeper slopes,* I feel sure that a sec- 

 tion of the stream in question would be such as is repre- 

 sented at No. 1, fig- 69, in which the central mass b would 

 be as compact and stony as the ordinary lavas of Somma 



materials 



The 



lower scoriae c is usually the least thick ; it is formed in part 

 by the sudden cooling of the lava pouring over the cold and 

 damp soil, while the upper bed a is that which consolidates 

 by contact with air. But as the 



mass 



an older lava, the layer c would join on to the upper scoria 

 of a subjacent stream, so that the solid and stony mass b 

 would be separated by a bed of considerable thickness from 

 the solid layer next below. The stream No. 2 next flows 

 down and has a similar structure, and to this succeeds the 

 third stream, No. 3, which fills up the interspace between 

 Nos. 1 and 2, and is also made up in like manner of three 

 parts ; a cross-section of the whole exhibiting a central, 



homo 



because all 



from 



fluid 



more 



the crater in 1857. Probably the beds of Monte Somma 

 seen in the Atrio at the bottom of the great section were 

 formed originally near the base of the cone, which had only 



ien attained a small part of its present dimensions, and 

 ie inclination there being less than 18°, some currents may 



* See paper by the author on the Structure of Etna, Phil. Trans. 1858, p. 734. 



