112 RAMBLES OF A NATURALIST. 



some of the laws which regulate their movements. It 

 will be easily understood that a liquid or viscous 

 mass will not comport itself in the same manner 

 on planes of different inclination. When rapidly 

 carried along a considerable inclination, such a fluid 

 is unable to form a layer of the same thickness as 

 it would attain on an almost horizontal surface. 

 "Wherever the inclination diminished the lava stream 

 would be deep, and wherever the inclination was 

 increased the stream would be shallow. Conse- 

 quently, in order that a stream of lava should pre- 

 sent an equal thickness over a considerable extent, 

 it is necessary that it should flow along a uniform 

 incline. 



When we examine lava beds with whose origin 

 we are acquainted, we always find that observation 

 is fully in harmony with theory. On slopes of great 

 inclination these lava streams only leave a narrow 

 and thin bed which is almost entirely composed of 

 scoriae, that is to say, of portions already solidified 

 by contact with the air, whilst they accumulate on 

 reaching gently inclined slopes, and here form thick 

 and compact strata. These very simple facts may 

 be verified by the modern streams which intersect 

 the mass of Etna in all directions, and we meet with 

 very frequent examples of this on the road from 

 Nicolosi to the crater, and a little above the Casa 

 del Bosco. 



Now when we penetrate into the Val del Bove, 

 that strange and celebrated valley which bears in- 

 scribed upon it, in ineffaceable characters, the history 

 of the formation of the volcano, we verify several facts 



