344 Association of American Geologists and Naturalists. 
how shall we derive from a vibration radiated from a deep focal spot or 
line, their remarkable number and isochronism? A simple vibratory. 
jar sent through the crust, if competent to produce a great wave at all, 
could on the hypothesis produce no more than one, as the result of a single 
concussive force, so that when these waves follow each other, at regu- 
lar intervals of a quarter of a minute or more, for several minutes, 
‘we must admit that they are generated in some other manner, since 
there is no conceivable cause for a strictly isochronous repetition of the 
subterranean force. 
But an objection of another kind suggests itself, in the excessively 
fissured and crushed condition of the strata in many regions, and their 
extremely heterogeneous composition, which must inevitably lead toa 
rapid gated or see: “P a a megoe, waves of vibration within 
the rocky ma 
An eminent t British eniegies pr Oe that the sndildionh mo- 
tion in earthquakes, may be of the nature of the vibration in a stretched 
cord when it is struck; but Prof. Rogers and his brother find it difficult 
trimnpginerihe es e deny the theory mis a pulsating fluid under the crust, how 
and so unelastic, nodal vibrations could 
take place i in the solid. fabric of the globe, causing waves of the heigl 
and een ati of the earthquake undulations. 
ecting the opinion that the vibratory jar is.the cause of se eos 
latory motion, they deem it more in accordance with known phenome- 
na, to recognize it as the effect, and to attribute the tremor to an exten- 
sive, minute fissuring and grinding together of the strata under the alter- 
nate dilatation and compression going on in wiely part of the ater 
mass, during the undulation. ave 
The dimensions of the individual inundations would eppannin beans 
ceptible in certain cases of direct calculation. Though the waves or 
temporary flexures must be of various magnitudes in different earth- 
quakes, their amplitude in the more violent convulsions is manifestly 
very great. Thus taking the data furnished by Darwin in his account 
of the earthquake of Conception, it may be shown that the probable 
width of each pulsation in that instance, was at least ten geographical 
miles, while there is reason to conclude that in the great Lisbon’ earth+ 
ange each wave of the crust had an amplitude atoinening to aes 
five miles. 
That the jwago-like snctiel of the earth is. of the chabchen af, an. ati 
al billowy pulsation, is shown not only by the visible heaving of the 
ground, but by the sensations produced, and by the alternate opening 
and closing of enormous. parallel chasms of great depth, and the direc 
tion of these, phich. i; iene to the course of the undulation. 
Of the manner in which the v in ear es 
a es 
