FORMATION OF LAVA 877 



of the earth's crust, the rise in temperature equal to that observed 

 in deep wells and mines would be produced by this cause alone, 

 irrespective of any other source of heat. Whether this distribution 

 is equal, or whether it increases or decreases downward, can not, 

 at present, be determined; but it is seen that if we start with an 

 original increase in temperature downward the amount added to 

 it by radio-activity might serve, locally, to overcome the opposing 

 effects of pressure in raising the fusing point, whereupon reser- 

 voirs of molten rock would be formed which w^ould become the 

 source of volcanic activity. 



If excessive temperature increase is not to be accepted as the 

 cause of rock fusion at a depth, we must turn to a local decrease 

 of pressure to permit the lowering of the fusing point of the rocks. 

 This would be effected by the formation, locally, of rock arches 

 within the crust capable of maintaining the weight of the superin- 

 cumbent portion of the crust. Such arches, or domes, by relieving 

 the pressure, would permit the liquefaction of the rock mass for 

 some distance beneath them, provided the temperature is sufficiently 

 high, and so furnish the requisite conditions for volcanic activities. 



Arches of this type might be expected to form along the margins 

 of the continent where the down-warping of the continental edges 

 takes place. Now, it is precisely along these lines, where the con- 

 tinental margin drops off steeply to the deep sea, that the great 

 volcanic phenomena of the past have been located, while the dis- 

 tribution of most of the modern volcanoes of the earth is essentially 

 in harmony with this idea. Thus by far the largest number of 

 still active or but recently extinct volcanoes are ranged in belts or 

 lines parallel to the margins of the continents or within the oceanic 

 areas. The most important belt of volcanic activity surrounds 

 the Pacific Ocean, the deepest and perhaps the oldest of the oceans 

 of the earth, and the one which has experienced the least change. 

 This belt includes the volcanic mountains of the west coasts of 

 South and Central America, of Mexico, and of the western United 

 States and Canada to Alaska and the Aleutian Island chain. It is 

 continued along the eastern coast of Eurasia, and through the 

 Malaysian islands to New Zealand, the belt being finally closed by 

 the volcanoes of Victoria Land, King Edward Island, and West 

 Antarctica. It is significant that the belt for the most part is 

 paralleled by an inner one of exceptional depressions the great 

 fore-deeps of the marginal Pacific. That these are produced by 

 downwarping or faulting seems certain, and this would imply an 

 arching or unwarping of the adjoining continental margins. 



