Mr. Mater on the Dynamics of Earthquakes. 61 
First, then, the original impulse may either be on land, as in a volcanic 
region, situated in the heart of a great continent or island, or it may be beneath 
the bed of the ocean. The former case is that of South America, where, in the 
voleanic region of the Andes, earthquakes are of almost daily occurrence, most 
of them, however, not of great power or extent. 
The latter case seems to be that, which almost universally belongs to great 
earthquakes; their origin is beneath the ocean, and that this should be so is probable 
when we consider that the force and extent of the shock must depend upon the 
resistance overcome by the original impulse, which will probably be always far 
greater when the crust of the earth is ruptured beneath the deep ocean, and un- 
der its profound masses of deposited materials, than on elevated land, where the 
already formed continents are provided with safety valves in the acting volcanoes. 
Now, when the original impulse comes from the land, an elastic wave is pro- 
pagated through the solid crust of the earth and through the air, and transmitted 
from the former to the ocean water, where the wave is finally spent and lost. 
When, on the other hand, the original impulse comes from the bed of the deep 
ocean, three sorts of waves are formed and propagated simultaneously, namely, one, 
or several successively through the land, which constitutes the true earthquake 
shock or shocks ; and coincident with, and answering to every one of these, one 
or more waves are formed and propagated through the air, which produce the 
sound like the bellowing of oxen, the rolling of waggons, or of distant thunder, 
accompanying the shock ; and a third wave is formed and propagated upon the 
surface of the ocean, which rolls in to land, and reaches it long after the shock 
or wave, through the solid earth, has arrived and spent itself. 
The plan, or horizontal outline of each of these waves, will be more or less 
circular or elliptical at first, according as the origin or centre of disturbance is 
at a single point, or along a line of impulse more or less regular, and the crests 
of the earthquake waves of every order of which we are about to speak, or, as 
they may be called, the earthquake cotidal dines, will, in their progress of pro- 
pagation through the earth and sea, alter their curvilinear forms, by changes in 
their respective velocities, becoming more and more distorted from the original 
form ; but, in every case, these cotidal lines will form closed figures. This will 
be evidenced to the eye by reference to our diagram of earthquake cotidal lines, 
(see Plate I.), the theory of which will be further explained hereafter, 
