280 PROCEEDINGS OF SECTION C. 



sufficiently cool for the mass to act as a solid a tendency to fracture 

 and to open a fissure would be followed by a movement of the slightly 

 more heated mass beneath to occupy the space between the fractured 

 solid ; these differences of temperature and of rigidity are to be under- 

 stood in a mathematical sense as differential, there being a gradation 

 of physical conditions between adjacent parts of the mass. There will 

 be no open space, or fissure, in the ordinary sense. But it must be 

 understood that at whatever depth the mass may be considered solid, 

 there it may fracture, part, and become the walls of a layer or body 

 of intruded liquid, provided the liquid has nearly the same density as 

 the solid mass. 



The statement made by Van Hise and Hoskins that open cracks, 

 or fissures, cannot exist at greater depths than about 10,000 meters 

 was made on the assumption that the filling is water ; the difference in 

 weight of the rock and the hydrostatic pressure of the corresponding 

 column of water being compared with the crushing strength of the 

 folid rock. When the liquid is heavier than water the same method 

 of calculation allows fissures filled with such liquid to exist at greater 

 depths; and if the weight of the column of liquid equals that of the 

 wall rock, the two will remain in equilibrium at any depth. Conse^ 

 quently, at any depth in the earth mass where a tendency to part may 

 exist, hotter and potentially more fluid material beneath may move 

 up and permit the parting of the slightly more rigid mass to take 

 place. This would appear to be the initial step in the eruption of rock 

 magma. 



As the mass shifts its position upward the pressure upon it 

 decreases, resulting in some expansion of the volume, some decrease in 

 density, some increase in mobility. A.nd the rising mass is hotter than 

 the masses between which it is rising, unless movement is at the same 

 rate as the diffusion of heat. In pi-oportion as the tensile stress is 

 strong the upward movement will be pronounced, and may result in a 

 flow of very dense, hot, viscous magma toward tlie surface of the earth. 

 The greater the vertical distance traversed and the more rapid the 

 rate of movement, the greater the difference in tenqierature between 

 the magma and the inclosing mass. 



That the eniption of rock magma is consequent upon the adjust- 

 ment of accumulated stresses within the overlying rocks is indicated 

 by the sequence of fractures and lava flows in the uppermost parts 

 of the earth, and the opening of eras of gi'eat volcanic activity after 

 profound orogenic movements have disturbed the comparatively quiet 

 action of forces that have been gradually shifting the stresses within 

 the outer portion of the earth. The magnitude of the adjusting action 

 is evinced by the extent of territory simultaneously affected. As, for 

 example, the initiation of volcanic action on a gigantic scale through- 

 out western America at the end of Cretaceous time, after an enormous 

 period of nearly vmiform conditions of comparative quiet. 



The eruptive impulse, or energy, causing the upward flow of 

 magma, must originate in the expansion of the magma upon relief of 

 pressure consequent upon the adjustment of stresses in the overlying 

 mass, and from expansive energy of dissolved gases. That the 

 eruptive force is of nearly the same order of magnitude as the stresses 

 within the earth's crust is shown by the relatively small amount of 

 niaterial erupted upon the surface of the earth compared with the 



