22 J. G. Goodchikl — Aug en- structure and Eruptive Roelis, etc. 



certainly aflfected tlie earth's crust, even at great depths from the 

 surface, the deeper-seated rocks must constantly tend to pass into 

 the molten condition, but are prevented from melting owing to their 

 fusing point being raised in consequence of the superincumbent 

 pressure. But if this superincumbent pressure is relieved while the 

 rocks are in the potentially molten condition referred to, they pass 

 at once into the molten state. The same result follows, of course, if 

 the temperature rises while the pressure remains constant. A relief 

 of pressure is thus equivalent in this case to a rise of temperature, 

 and is, for several reasons, the more likely cause of fusion. 



Once these rocks are reduced to the molten condition they tend to 

 eat their way upward in any direction of least resistance — the place 

 of the material flowing upwards being at first taken chiefly by the 

 colder masses of rock, which sink within the magma as fast as they 

 are quarried from the sides of the vent. On arriving within a few 

 miles of the surface the heated rock occludes HjO and is thereby 

 rendered more fluid, and at the same time receives an additional 

 impulse through the elastic force exerted by the included gases. 

 These finally impel the molten rock to the surface, and when the 

 communication between the plutonic zones of fusion and the surface 

 of the earth is finally established, a volcano is the result. 



The products of dynamo-metamorphism may not reach the surface, 

 but may be arrested at any horizon between their starting-point and 

 the surface. Volcanic products therefore graduate downward through 

 the trappean into the plutonic rocks, which, in their turn, terminate, 

 at great depths, against the zones of dynamo-fusion. 



When once volcanic action has commenced, the place of the 

 ejected material is partly taken by the downward subsidence of the 

 A'olcanic centres, partly filled in by a lateral flow of rock-material 

 towards the deeper parts of the foci. As this movement itself takes 

 place under enormous pressure, it not only tends to produce schist- 

 osity in the rocks within the zones afiected, but it must at the same 

 time give rise to a further evolution of heat. 



Such results as those just noticed follow when the relief of 

 pressure upon a potentially molten mass acts per salhim. This mode 

 of relief must often come into action, more especially in connection 

 with the subterannean explosions, which are due to the sudden 

 access of water to greatly heated masses, or to the sudden relief 

 afforded when the tension of the vapour thus imprisoned overcomes 

 the resistance. But in quite as many cases, more especially within 

 the deeper zones of the earth's crust, the relief of pressure is directly 

 connected with the translation of the great terrestrial undulations 

 from one part to another. Such undulations, as a rule, may be 

 supposed to travel at very slow rates, so that it may require an 

 interval of many thousands of years for an upward phase of an 

 undulation to pass into a phase of movement in the opposite direction. 

 Assuming that difl'erential compression of the rocks acted upon is 

 one of the results of all such undulatory movements, it would follow 

 that, by slow degrees, the locus of greatest pressure travels with the 

 undulation, and gives place, also by very slow degrees, to one in 



