88 Mr. Matter on the Dynamics of Earthquakes. 
temperature due to depth. ‘Thus, then, granting the whole of Michell’s hypo- 
thesis, we find the inevitable results of his own theory will not square with 
observed facts on the one hand, nor with the decrees of physics upon the other. 
It would be, therefore, needless to discuss more at length the gratuitous and 
unsustainable character of his hypothesis as to the mode of production of the 
original impulse itself. 
It remains, then, to explain a little more fully the precise nature of the great 
earth wave of elastic compression assigned by me as constituting, by its transit, the 
earthquake shock, and to explain briefly the general mode of origination of the 
impulse by which it is produced, or at least may be conceived to be produced, 
without the assumption of any hypothesis beyond the established facts, that forces 
of some kind, acting from below upwards, produce local elevations of portions of 
the earth’s solid crust, often attended with dislocation and fracture of the crust, 
and sometimes attended with the actual outpouring of liquid matter from beneath ; 
that these elevations take place with various degrees of rapidity, sometimes con- 
tinuing to lift the land slowly for many years, as in Norway and Sweden; at 
other times producing an upheaval of several feet in a very short time, and that 
such elevations occur both on land and beneath the ocean. 
In such local elevations, then, I find the efficient cause of the earthquake 
shock, which I define to be a wave of elastic compression, produced either by 
the sudden flexure and constraint of the elastic materials forming a portion of 
the earth’s crust, or by the sudden relief of this constraint by withdrawal of the 
force, or by their giving way, and becoming fractured. 
When a portion of the solid crust of the earth, whether consisting of strati- 
fied or of igneous rock, is urged by forces of elevation from below (of whateve 
sort), having a certain definite elasticity and flexibility, the whole plate of solid 
material above may be viewed as a platform or beam, supported or held fast 
at the edges or ends, and loaded or pressed upwards, more or less uniformly, from 
beneath. If the platform or beam bend under the strain, all the particles 
below a certain neutral plane will be thrown into a state of compression, those 
above it into the opposite condition of extension ; and if this bent and constrained 
condition of the plate or crust be suddenly produced—if the pressure from below 
be suddenly brought upon it, a wave of elastic compression will, at the moment 
of flexure, be produced, and propagated at once in every direction outwards 
from the fixed edges of the plate, or through the surrounding portions of country 
