372 GEOLOGY 



extinct volcanoes show that the former passageways, leading 

 down toward the sources of the lava, vary much in size and shape, 

 and usually have diameters smaller than the craters. 



The exact number of volcanoes now active cannot be stated 

 precisely, because most volcanoes are in action only at more or 

 less distant periods, and it is impossible to say whether a volcano 

 that is now quiescent is extinct or only resting. It is quite safe 



Fig. 291. Cinder cone forming the summit of Mt. Vesuvius 



to include at least 300 in the active list, and the number may 

 reach 350 or more. The number that have been active so recently 

 that their cones remain distinct is several times as great. 



Distribution of Volcanoes 



1. In time. In the earliest known ages, igneous action appears 

 to have been very widespread. No great area of the oldest 

 (Archaean) rocks is now known where the formations are not largely 

 igneous, either of the intrusive or of the extrusive kind. From 

 the Paleozoic to the present, the distribution of volcanic action 

 over the surface seems to have been, in a general way, much what 

 it is to-day; that is, certain areas were affected at times by vol- 



