870 



PRINCIPLES OF STRATIGRAPHY 



cone contrasts strongly with the concave surfaces of the cinder 

 cones. The results of experiments recorded in Figures 228 and 

 2^^ show the fundamental ditiferences hetween fragmental cones 

 and domes of pustular lava. 



The spine of Pclce. What is regarded by many as a most 

 stupendous example of the extravasation of a viscous mass of 

 andesitic lava — which cooled as it was extravasated — is found in 

 the remarkable spine of Mont Pelee which formed after the great 

 eruption of 1902. According to Hovey (16; 17; 18), this spine 

 was a lava mass pushed up vertically without spreading, the mass 

 cooling either in the upper part of the conduit or upon its appear- 

 ance at the surface, so that no extended flow was possible. The 

 spine grew at an average of forty-one feet per day during a period 



Fig. 232. The Grand Puy of Sarconi, 

 in the Auvergne, composed of 

 trachyte, rising between two 

 breached scoria-cones. A typical 

 example of a pustular cone 

 formed of highly viscid lava. 



Fig. 233. Experimental illustration 

 of the mode of formation of vol- 

 canic cones composed of viscid 

 lavas. 



of eighteen days out of the new cone, which itself had attained a 

 height of 1,600 feet during the last ten days of May, and was of 

 the same character as the spine. As the spine rose 1,100 feet above 

 this new cone in October it appears that the total elevation of this 

 mass above the top of the cone as it existed prior to the eruption 

 of May, 1902, was 2,700 feet. (See. further, Heilprin-12; Hill- 

 12a; Jaggar-20 and Russell-34a.) 



Composite Lava and Cinder Cones. 



Volcanic cones built by a combination or an alternation of the 

 explosive and extravasative activities are by far the most common. 

 They generally have pronounced slopes and are more resistant than 

 cones built wholly of cinders, because the lava binds together the 

 loose material into a complex mass. This is sometimes accomplished 

 by the formation of radial dikes, as in the case of ^tna. These 

 represent lateral Assuring of the cone and the filling of these fissures 



