THE EXTRUSIVE PROCESSES. 599 



VOLCANOES. 

 Number of volcanoes. — It is impracticable to state exactly the num- 

 ber of volcanoes that are active at the present time, because most vol- 

 canoes are periodic, and become active at more or less distant periods, and 

 it is impossible to say whether a given volcano that may be now qui- 

 escent has really become extinct or is only enjoying its customary period 

 of rest. It is quite safe to include at least 300 in the active list, and the 

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

 recently that their cones have not been entirely worn away is several 

 times as great. 



Distribution of Volcanoes. 



1. In time. — In the earliest kno\\Ti ages igneous action appears to 

 have been very general, if not practically universal. No area of the 

 earhest (Archean) rocks is now kno\vn which is not formed chiefly of 

 rocks that appear to have been either intruded or extruded. Rocks 

 which can reasonably be assigned to the hypothetical molten globe, if 

 there be such, are not here included. It is probable that the surface 

 of the early earth was as thicM^ occupied with points of extrusion as 

 the surface of the moon appears to be. In the ages between the Archean 

 and 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 volcanically active at times, while other and larger 

 areas were measurably free from any outward expressions of igneous 

 action. This is not equally true of all ages, as will be seen in the his- 

 torical studies that follow. There were periods when volcanic activity 

 seems to have been widespread and energetic, and others when it was 

 limited both in amount and distribution. The known facts do not 

 indicate a steady decline in volcanic activity, but rather a periodicity; 

 at least this is so for the portion of the globe that is now well enough 

 known geologically to warrant conclusions. One of the greatest of the 

 volcanic periods falls within the Cenozoic era, just preceding the present 

 geological period, and the volcanic activity of the present is perhaps 

 but a declining phase of that time. 



2. Relative to land and sea. — At present the active volcanoes are 

 chiefly distributed about the borders of the continents, and, less notably, 

 within the great oceanic basins. On this account the sea has often 

 been supposed to have some connection with volcanic action, and the 



