342 Association of American Geologists and Naturalists. 
at Valdavia, according to the interesting description of Capt. Fitzroy, 
was undulating and regular, like waves rolling, from west to east, but 
strong; and it lasted nearly ten minutes. The houses waved and crack- 
ed. The same earthquake was felt at the island of Chiloe by Mr. 
Douglas, who describes the motion, we are told by Mr. Darwin, as hor- 
izontal and slow, similar to that of a ship at sea, going before a high 
regular swell, with three to five shocks in a minute. 
Prof. Rogers proceeded to show the manner in which these earth- 
quake undulations advance, and adduced some facts from which even 
the amplitude of the individual waves or pulsations may be approxi- 
mately computed. As a confirmation of the truth of the generalization, 
long ago arrived at by Michell, that the disturbance is not simultaneous 
over the whole region shaken, but is transmitted with a high velocity, 
he presented the results of an analysis of the earthquake which happen- 
ed in the United States, on the 4th of January of the present year, re- 
specting which he had collected some instructive information. 
This earthquake was felt from the sea-coast of Georgia and South 
Carolina, to beyond the western frontier military posts, and from the 
latitude of Natchez to that of Iowa, a space in each direction of about 
eight hundred miles; and there are reasons for believing that its actual 
extent was considerably greater. A comparison of the dates of the 
shock, as felt at the numerous localities heard from, seems to settle 
with satisfactory accuracy, the direction, velocity and mode of progress 
of this earthquake. The facts collected from more than twenty five 
stations, and embodied in a tabular form, make it obvious that the shock 
was simultaneous throughout an elongated and narrow belt or line, rang- 
ing in aN. N. £. direction from the western edge of Alabama, nearly 
through Nashville and Cincinnati, being also simultaneous along every 
other line having with this a parallel direction; whereas no such syn- 
chronism existed, where the localities compared were situated in any 
other than a N. N. E. and s. s. w. position. Places lying to the w. N. w. 
of others invariably encountered the convulsion soonest, and by an in- 
terval of time, in this case, strictly proportionate to the distance. — 
These general facts justify, it is conceived, two important conclu- 
sions ; first, that the area in agitation at any given instant was dinear, 
and secondly, that the earthquake moved from w. N. w. to E. s. E., keep- 
ing parallel to itself in the manner of an advancing wave. The data 
brought together in the table, indicate with considerable precision the 
velocity with which the shock was propagated from w. N. W. to E. S. E. 
Ascertaining the moment when the belt of synchronal disturbance reach- 
ed St. Louis, and that again when it coincided with a line passing through 
Tuscaloosa, Nashville and Cincinnati, the earthquake is found to have 
occupied about eight minutes and twenty four seconds in the transit, the 
