Febkuaey 5, 1909] 



SCIENCE 



215 



such a position must be in a virtually static 

 condition until it experiences change of 

 pressure or stress. Whatever its composi- 

 tion, it must remain unchanged. 



A change of stress may come about by 

 movement in the overlying portion of the 

 earth. Orogenie movement, readjustment 

 of the upper rigid, rock mass, from what- 

 ever causes, when profound, must affect the 

 stresses in still deeper parts. The known 

 erustal movements behave as bendings of 

 the upper rock mass, which in places at 

 the earth's surface appear to result in ten- 

 sile stresses; in places, in compressional 

 stresses. Beneath each of these the effect- 

 ive stresses must be of the opposite kind; 

 under the tensile, compressive stresses, and 

 under the upper compressive ones, tensile 

 stresses. Tensile stresses should occur at 

 some distance below ocean beds, and more 

 especially along the borders of oceans and 

 continents. Compressive stresses should 

 occur, in general, beneath continental 

 masses. 



Tensile stress, as at the bottom of a 

 synclinal arch, operating in a rigid mass 

 must communicate itself downward as far 

 as the mass behaves rigidly. When the hot 

 mass is potentially fluid, that is, is kept 

 solid by pressure, change of stress must be 

 followed by change of position of the mass. 

 A tendency to pull apart or stretch in the 

 potentially fluid mass must be followed by 

 a yielding of the mass. At a point suffi- 

 ciently 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 oc- 

 cupy the space between the fractured solid ; 

 these differences of temperature and of 

 rigidity are to be understood in a mathe- 

 matical 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 be- 

 come the walls of a layer or body of in- 

 truded liquid, provided the liquid have 

 nearly the same density as the solid mass. 

 The statement made by Van Hise and 

 Hoskins that open cracks, or fissures, can 

 not 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 pres- 

 sure of the corresponding column of water 

 being compared with the crushing strength 

 of the solid rock. When the liquid is 

 heavier than water the same method of cal- 

 culation 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. Consequently 

 at any depth in the earth mass where a 

 tendency to part may exist, hotter and po- 

 tentially 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 de- 

 crease in density, some increase in mobility. 

 And the rising mass is hotter than the 

 masses between which it is rising, unless 

 movement is at the same rate as the dif- 

 fusion of heat. In proportion as the ten- 

 sile stress is strong the upward movement 

 will be pronounced, and may result in a 

 flow of very dense, hot, viscous magma to- 

 ward the surface of the earth. The greater 

 the vertical distance traversed and the more 

 rapid the rate of movement, the greater the 

 difference in temperature between the 

 magma and the enclosing mass. 



That the eruption of rock magma is con- 



