520 ON THE PHYSICAL STRUCTURE 



no cohesion between one stratum and another. The part of the 

 earth that is first raised, being bent from its natm-al form, will 

 endeavor to restore itself by its elasticity, and the parts next to 

 it being to have their weight supported by the vapor which will 

 insinuate itself under them, wiU be raised in their turn, tiU it either 

 finds some vent, or is again condensed by the cold into water, 

 and by that means prevented from proceeding any fm-ther."* 



Now we conceive that there is a simpler view of the origin of 

 the undulation, and one which is more in accordance with sound 

 dynamical considerations, and with all the recorded observations 

 upon earthquakes. In place of supposing it possible for a body 

 of vapor or gaseous matter to pass horizontally between the 

 strata, or even between the crust and the fluid lava upon which it 

 floats, and with which it must be closely entangled, we ai-e in- 

 chned to attribute the movement to an actual pulsation, engen- 

 dered in the molten matter itself, by a linear disruption under 

 enormous tension, giving vent, explosively, to elastic vapors, 

 escaping either to the surface, or into cavernous spaces beneath. 

 According to this supposition, the movement of the subterranean 

 vapors would be towards, and not from the disrupted belt, and 

 the oscillation of the crust would originate in the tremendous and 

 sudden disturbance of the previous pressure on the surface of the 

 lava mass below, brought about by the instantaneous and violent 

 rending of the overlying strata. 



It has been denied by some — and the objection seems to be 

 acceded to by Mr. Lyell — that the so-called wave-Ulce motion of 

 the surface in earthquakes, has " any stiict analogy with the un- 

 dulations of a fluid." On the other hand, " it has been suggested, 

 that a vibratory jar may be produced at a considerable depth, by a 

 sudden fractm-e of the solid crust, and tliat the vibrations may be 

 propagated upward through a mass of rock, even several miles 

 thick. The flrst vibration which reaches the surface will lift the 

 soil, and then allow it to sink again ; immediately after which 

 another, which may have radiated from the same deep-seated 

 point, may arrive at a contiguous spot on the surface, and cause 



* Michell, Phil. Transactions, 1760. 



