128 



EARTH FEATURES AND THEIR MEANING 



they increase the dimensions of the crater, the Vulcanian stage in 

 contrast to the other has been called the crater-producing period. 

 In this period, then, the material ejected during the explosions does 

 not consist solely of fresh lava cakes, but in part of the older debris 

 derived from the crater walls, whence it is avalanched upon the 

 chimney after each larger explosion. The over- 

 hanging cloud, which during the Strombolian 

 period has consisted largely of steam and is 

 noticeably white, now assumes a darker tone, 

 the " smoke " which characterizes the Vulcanian 

 eruption. 



On several historical occasions the cone of 

 Vesuvius has been lowered by several hundred 

 feet, the greatest of relatively recent truncations 

 having occurred in 1822 and in 1906. Between 

 Vulcanian eruptions the Strombolian activity is 

 by no means uniform, and so the upward growth 

 of the cone is subject to lesser interruptions and 

 truncations (Fig. 129). 



The Vesuvian eruption of 1906 has been 

 selected as a type of the larger Vulcanian erup- 

 tion of composite cones, because it combined the 

 explosive and paroxysmal elements, and because 

 it has been observed and studied with greater 

 thoroughness than any other. The latest pre- 

 vious eruption of the Vulcanian order had 

 occurred in 1872. Some two years later the 

 period of active cone building began and pro- 



fications in its out- 

 line (after Sir Wil- 

 liam Hamilton). 



FIG. 129. A series 

 of consecutive 

 sketches of the 

 summit of the 



Vesuvian cone, ceeded with such rapidity that by 1880 the new 

 showing the modi- cone began to appear above the rim of the crater 

 of 1872. From this time on occasional light 

 eruptions interrupted the upbuilding process, 

 and as the repairs were not in all cases com- 

 pleted before a new interruption, a nest of cones, each smaller 

 than the last, arose in series like the outdrawn sections of an old- 

 time spyglass. At one time no less than five concentric craters 

 were to be seen. 



For a brief period in the fall of 1904 Vesuvius had been in almost 

 absolute repose, but soon thereafter the Strombolian crater ex- 



