102 Mr. Matter on the Dynamics of Earthquakes. 
The lines of junction of these groups are indicated by a strong, dotted line, coloured 
according to the formation at either side thereof. 
The centre of impulse, or of earthquake disturbance, is supposed to lie under the bed 
of the sea, at a point directly beneath the intersection of the sections of soundings. 
The dark, continuous, and nearly circular curves, represent the crest of the earth wave 
of shock, at successive small intervals of time, as it traverses each of the formations of the 
map, starting from the centre of impulse; and the fine dotted curves represent the crest of 
the great sea wave, at similar successive small intervals of time, in its progress towards 
the surrounding coasts. The surface lines of the sounding sections thus may be viewed 
as ordinates of time, along which these wave crests may be supposed to move. 
On examining the cotidal curves for the earth wave, it will be remarked that, start- 
ing from a single point in the upper sedimentary rocks, they are perfect closed circles, so 
long as they continue in the original formation; but as soon as they pass the junc- 
tion at one side, they begin to move faster, or have a larger space interval in the 
crystallized and more elastic lower sedimentary rocks, while, at the other side, having 
penetrated into the superficial formations (post-tertiary and tertiary) of lower elasticity, 
they move more slowly, and with a less space interval. Again, when passing from the 
lower sedimentary into the igneous rocks, they reach a formation of still higher elasticity, 
the earth wave crest shows a still greater augmentation of velocity. At the junction of 
the several formations, therefore, the earth wave curve suffer a sudden flexure, produced 
by the sudden change of velocity ; and at these points the direction of transit of the 
earthquake shock not only differs from that at either side of the junction, but continually 
changes along the same line of junction in the progress of the wave. Thus, the earth 
waye, at the interval LV., traverses the junction of the upper and lower sedimentary for- 
mations (to the south-east of the map), in a direction nearly due south; but as it passes 
through the same junction at the intervals V., VI., VII., VIII., 1X., X., and XL, it 
continues to pass through successive points of the compass, until, at X. and XL., its 
transit is made in a direction nearly north-west, and almost at right angles to the direction 
of shock, closely adjacent, within the two formations at either side of the junction. Other 
similar apparent anomalies will be obvious, upon a careful inspection of the earth wave curves 
on other parts of the map. These curves, of course, only represent the crest of the princi- 
pal earth wave. At every sudden flexure there will be many minor divergent waves. 
Referring, now, to the (dotted) cotidal curves of the great sea wave, it will be remarked 
that, as the progress of this wave depends upon a function of the depth, so the originally 
closed and nearly circular curves of the great sea wave become more and more distorted, 
by its transit through the variable depths of the sea, moving with great rapidity in the 
deep water, and here showing a large interval, but moving more and more slowly as the 
water shallows near the coast and in the estuaries. 
It results from this, that the direction of the great sea wave, on its arrival at a dis- 
