THE STRENGTH OF THE EARTH'S CRUST 439 



pyrosphere has its roots in the asthenosphere. But in the attempt 

 to frame a logical picture of the processes which determine the 

 ascent of magmas, the question arises how diffused liquid matter is 

 drained away, rising and uniting at higher levels into magma 

 reservoirs, temporarily molten, and the direct source of the igneous 

 activity exhibited within the outermost crust. A deductive 

 picture is as follows — one whose truth cannot be tested directly, 

 but only by its general agreement with our understanding. 



At the place of origin, liquid of an andesitic or basaltic nature 

 will come to honeycomb the rock. The content of gases is pre- 

 sumably sufficiently high to reduce the viscosity. The liquid 

 will then become able to transmit hydrostatic pressures, and, 

 although comprising only a part of the rock mass, will constitute 

 a continuous column of considerable height. Then becomes 

 possible the second stage, the draining upward and the convergence 

 of the fluid rock. Gravity is the ultimate cause, as in the down- 

 ward movement of waters, but here the fluid, being lighter than its 

 surroundings, tends to move upward. An explanation of this 

 draining process has been given by Lane.^ In a gas-saturated 

 rock an excess of gas, or liquid and gas, would have the power of 

 opening fissures at any depth in the zone of flow without the 

 necessity for the existence of any tensile stress in the walls, or 

 competence in the walls to maintain an open cavity. All that is 

 necessary is that the excess pressure in the rising wedge of gas 

 should be stronger than the cohesion of the rock. The fissure 

 becomes filled with gas and fluid of lesser density than the solid 

 rock of the walls. Consequently, the pressure transmitted from 

 below is greater than the resisting pressure in the walls. This 

 insinuating power, owing to the hydrostatic head due to the lesser 

 gravity of the wedge, becomes greater the higher the wedge rises 

 above the source, until near the surface the action may become 

 violent and rapid. Daly also has outlined a theory of mechanism 

 for the injection of abyssal wedges of magma into the upper crust.^ 



^"Geologic Activity of the Earth's Originally Absorbed Gases," Geological 

 Society of America Bull., V (1894), 259-80. 



^ Am. Jour. Sci., XXII (1906), 195-216; Igneous Rocks and Their Origin (1914), 

 chap. ix. 



