Association of American Geologists and Naturalists. 345 - 
be supposed to originate, Michell suggests, that large tracts of country 
may be supposed to rest on fluid lava, which when disturbed may trans- 
mit its motion through the overlying crust. Comparing the process to 
the waves caused in a carpet, when one end of it is lifted from the floor 
and suddenly brought down again, he conceives, that ‘+a large quantity 
of vapor may raise the earth in a wave, as it passes along between the 
‘strata, which it may readily separate in a horizontal direction, there be- 
ing little or no cohesion between one stratum and another. The part 
of the earth that is first raised, being bent from its natural form, will en- 
deavor to restore itself by its elasticity, and the parts next to it having 
1eir weight supported by the vapor which will insinuate itself under 
them, will be raised in their turn, till it either finds some vent or is 
again condensed by the cold into water, and by that means prevented 
from proceeding any further.” pio 
Prof. Rogers and his brother propose an explanation of the origin of 
the pulsation which they deem more in harmony with sound dynamic. 
considerations, and with the observed phenomena of earthquakes. 
stead of supposing it possible for a body of vapor to pass horizontally 
between the strata, or even between the crust and the fluid lava, which 
at their contact must be closely entangled, they attribute the movement 
to an actual pulsation in the molten matter itself, engendered by a lin- 
ear disruption of the crust from enormous tension, and the sudden or 
explosive escape of highly compressed steam and gaseous matter. ‘Up- 
on this doctrine the course of the subterranean vapors would be towards 
and not from the line of disruption, and the undulation of the crust 
Would arise from the instantaneous and excessive change in the press- 
ure on the surface of the lava mass, the operation of which would be 
as effectual as a sudden downward stroke in creating in the fluid a sys- 
tem of great oscillatory waves. The billows excited on the surface of 
the sea of molten lava, by the rupturing and immediate collapsing: of 
the crust, must, it is conceived, be of the nature of progressive waves of 
oscillation. Generated in a group on each side of the axis of disturb- 
ance, these waves will move off in parallel order, the two belts coales- 
cing at their extremities to form a rapidly dilating elliptic zone, the out- 
line of which will mainly depend on the form and elongation of the rent. 
Around the extremities of the fissure, the pulsation will be feeble from 
the rapid reabilien progress, in this position of the Eres and this per- 
haps may explain the absence of a sensible shock during the Guada- 
loupe earthquake, in the region N. and N. w. of Bermuda, while it was 
distinetly felt to a great distance in a due west direction. If the earth’s 
crust be ruptured along a very short line, or the rent be by the orifice 
of a volcano, the pulsation will be approximately circular. Such seems 
to have been nearly the form of the celebrated Lisbon earthquake. 
Vol. xuv, No. 2.—July-Sept. 1843. at 
