F. A. Per ret — Vesuvius. 



425 



4. The mud flows, as a destructive post-eruptive phe- 

 nomenon, have formed a conspicuous feature of the repose- 

 period. The eruption left the mountain covered with deep 

 layers of sand in various degrees of fineness mingled with 

 blocks and bowlders of every description, and, although the dry, 

 hot avalanches during the paroxysm had carried much of this 

 material off the cone, this only caused its removal farther down 

 the mountain and thus nearer to inhabited parts. The rains 

 which follow an eruption, and those of succeeding rainy seasons, 

 seep through the sand until a certain consistency is reached, when 



Fig. 9. 



Fig. 



Vesuvius — Water erosion on the mud-flows. 



the mass begins to flow as a mud stream carrying along the 

 blocks and bowlders and acquiring considerable velocity. Fol- 

 lowing the ravines and gullies on the flanks of the mountain, 

 the "mud-lava" invades the plains below, causing the destruc- 

 tion of houses, bridges and, not infrequently, of human life. 

 The government has constructed, at great expense, a series of 

 stone dams designed to impede the flows, and these have, on 

 the whole, served the purpose fairly well. There now remains 

 comparatively little of this movable material and the flows most 

 often seen at present are of the self- arresting type, having the 

 general shape of a glacier. 



The movement of this material under the varying action of 

 meteoric waters forms an interesting field for the study of 

 denudation and drainage, the accumulations on gentle gradients 



