134 



EARTH FEATURES AND THEIR MEANING 



advances without burning 

 the objects in its path 

 (Fig. 137). The beautiful 

 pines are merely charred 

 where snapped off and 

 are carried forward upon 

 the surface of the stream 

 (Fig. 138). When a real 

 obstruction, such as a 

 bridge or a villa, is en- 



FIG. 139. Lava front both pushing over and , ,, 



running around a wall which lie, athwart its Countered, the Stream IS 

 course (after Johnston-Lavis) . at first halted, but the 



rear crowding upon the 



van, unless a passage is found at the side, the lava front rises 

 higher and higher until by its weight the obstruction is forced to 

 give way (Figs. 139 and 140). 



The sequence of events within the chimney. The thorough 

 study of this Vesuvian eruption has placed us in a position to infer 

 with some confidence in our conclusions the sequence of events 

 within the chimney and 

 crater of the volcano, both 

 before and during the erup- 

 tion. Anticipating some 

 conclusions derived from the 

 observed dissection of vol- 

 canoes, which will be dis- 

 cussed below, it may be 

 stated that what might be 

 termed the core of the com- 

 posite cone the chimney 

 is a more or less cylin- 

 drical plug of cooled lava 

 which during the active 

 period of the vent has an 

 interior bore of probably variable caliber. This plug in its 

 lower section appears in solid black in all the diagrams of Fig. 

 141. During the cone-building period (Fig. 141 a and 6) the plug 

 is obviously built upward along with the cone, for lava often flows 

 out at a level a few hundred feet only below the crater rim. By 



FIG. 140. One of the villas in Boscotrecase 

 which was ruined by the Vesuvian lava flow 

 of 1906. The fragments of masonry from 

 the ruined walls traveled upon the lava 

 current, where they sometimes became 

 incased in lava. 



