VESUVIUS IN ERUPTION 61 



inner mass of the globe is still a matter of discussion, 

 theory and verification ; but whatever conclusions geolo- 

 gists may reach on these matters, the main fact of 

 importance is that steam and gases issue from these 

 fissures with enormous velocity and pressure, and that 

 "a vent" of this kind, once established, continues, as a 

 rule, to serve intermittently for centuries, and, indeed, for 

 vast periods to which we can assign no definite limits. 

 The solid matter ejected becomes piled up around the 

 vent as a mound, its outline taking the graceful catenary 

 curves of rest and adjustment to which are due the great 

 beauty of volcanic cones. The apex of the cone is blown 

 away at intervals by the violent blasts issuing from the 

 vent, and thus we have formed the " crater," varying in 

 the area enclosed by its margin and in the depth and 

 appearance of the cup so produced. At a rate depending 

 on the amount of solid matter ejected by the crater, the 

 mound will grow in the course of time to be a mountain, 

 and often secondary craters or temporary openings, 

 connected at some depth with the main passage leading 

 to the central vent, will form on the sides of the mound 

 or mountain. Sometimes the old crater will cease to 

 grow in consequence of the blocking of its central vent 

 and the formation of one or more subsidiary vents, the 

 activity of which may blast away or smother the cup-like 

 edge of the first crater. 



Such a history has been that of Vesuvius shown in 

 outline in Fig. 31. In geologic ages perhaps some 

 thousands of centuries ago Vesuvius was probably a 

 perfect cone (its outline is shown at the bottom of p. 62) 

 some 7000 ft. high, rising by a characteristically ac- 

 celerated upgrowth from a circle of ten miles or more 

 in diameter to its delicate central peak, hollowed out 

 at the summit by a small crater a couple of hundred 

 yards across. Its eruptions at that time were neither 



