DISTRIBUTION OF VOLCANO I > 



231 



canoes show that the former passageways leading down toward the 

 sources of lava vary much in size and shape. 



The exact number of volcanoes now active is not known, because 

 most volcanoes are active periodically only, and it is impossible to 

 say whether a volcano which is now quiescent is extinct or only 

 resting. It is safe to include 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. 



flavna Loa 



Fig. 225. Profile of the cone of Mauna Loa. 

 (U. S. Geol. Surv.) 



3oa/t S fTtte* 



Vertical scale same as horizontal. 



Fig. 226. Sketch of the crater of cinder cone near Lussen Peak, Cal., showing 

 tlie peculiar feature of two rings. The funnel is 240 feet deep. (U. S. Geol. Surv.) 



Distribution, i. In time. In the earliest known ages, igneous 

 action appears to have been very widespread. No great area of 

 the oldest (Archean) rocks is known where the formations are not 

 largely igneous. From the Paleozoic to the present, the distribu- 

 tion 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 volcanoes, while other and larger areas had few 

 or none. This is not equally true of all periods, as will be seen in 

 the historical studies that follow. There were periods when vol- 

 canic activity was widespread and energetic, and others when it was 

 limited. The known facts do not indicate a steady decline, but 

 rather a periodicity; at least this is so for the portion of the globe 

 that is now known well enough to warrant conclusions. 



2. Relative to land and sea. Active volcanoes are located 

 chiefly along the borders of continents, and within great oceanic 

 basins (Fig. 227). On this account, the sea-water was formerly 

 supposed to have some causal connection with their activity, and 



