78 Mr. Matter on the Dynamics of Earthquakes. 
where the sea is shallow and the shore shelving, be at first attended by one 
great sea wave, which, on coming within soundings, may divide into several, 
according to the height of the original wave and the depth of the water ; 
these secondary waves shall arrive in succession upon the land, and each earth 
wave will be followed by a similar sequel of divided sea waves ; or, if the depth 
of water close into the shore be sufficient, in proportion to the height of the 
original wave, although the latter be shelving or beached, it may come in singly, 
and only altered from its form when passing over the deep ocean, by having 
become steep and impending on its front slope until it breaks upon the land.* 
Thus, then, we havea complete account rendered of all the apparently perplexed 
facts recorded in the narratives of earthquakes, as to the occurrence sometimes 
of a number of shocks, and but one great wave ; sometimes of many shocks and 
several great waves; and sometimes of fewer great waves than shocks. 
It may be here remarked, that although the earth wave and the great sea wave 
have a common origin, and set out with motion in the same direction, that it by 
no means follows that they shall both arrive at land with precisely the same 
direction of motion ; for inspection of the map of cotidal earthquake lines (see 
plate I., and Explanation of the Plates) will shew that, as the earth wave cotidal 
lines become distorted by change of strata, or of geological formation, so the 
great sea wave cotidal lines become distorted by change in the depth of water, 
and hence, may intersect at one or at several points; and thus the shock of an 
earthquake may appear to come from one direction, while the great wave may 
rollin apparently from a different one, although they both started from one point, 
and at the one moment, which again explains some of the most perplexing accounts 
given in earthquake narratives. 
Just before the great sea wave comes in, the sea will largely and rapidly 
retire from the shore. Again, after the primary great sea wave has come on 
shore, the sea will, upon its retreat, fall back far beyond its usual limits; and again 
returning, a second wave may follow the first, of less size; and again, a third 
or a fourth, until these waves of oscillation have spent themselves. The possi- 
bility and extent of this phenomenon depend upon the form and slope of the 
beach, or shape of the bottom of the shallow water close in shore. This was 
* See Caldcleugh’s Account of the Earthquake at Conception, in 1835, Phil. Trans., where 
the wave was twenty-eight feet high. 
