218 
THE GEOLOGIST. 
of the xlppalachian Mountains. According to this theory, the 
wave-like motion of the earth's surface during an earthquake is 
of the nature of an " actual billowy pulsation in the molten 
matter/' upon which they suppose ths crust of the eaj-th floats, 
'^engendered by a linear or focal disruption and immediate col- 
lapse of the crust, accompanied by the explosive escape of highly 
elastic vapour."" The progressive waves of oscillation thus de- 
veloped on each side of the axis of disturbance, would move off 
in parallel order, and form a dilating elliptic zone. Supposing 
the earth's crust to be ruptured only at a focal point, as in the 
orifice of a volcano^ the receding pulsations would be approxi- 
mately circular-, whereas, if the line of fracture were greatly 
elongated, and the pulsations observed but on one side, the 
advancing belt of waves would appear straight. 
The sea-waves caused by earthquakes are described as broad 
undulations of the water, moving in the same direction with the 
pulsation of the crust beneath, at the rate of three and a half 
miles per minute in the case of the New England earthquake of 
1756 j and at the rate of five miles per minute dui'ing the great 
Lisbon earthquake, the waves succeeding each other at regular 
intervals of five minutes. Assuming these sea-waves to corres- 
pond with the undulations beneath, the authors calculate the 
breadth of the crust waves in the Lisbon earthquake at twenty 
five miles ; and in the earthquake of Conception, where the un- 
dulations averaged four per minute, and travelled forward at the 
rate of forty-two miles in the same time, each wave possessed a 
breadth of at least ten geographical miles. The tremor attending 
earthquakes is considered as the effect of the crushing of the 
strata during, and caused hy the undulations, rather than as the 
result of waves of vibration, which would be dispersed and des- 
troyed by the broken condition of the materials and their 
heterogenous composition. 
Application of the theory of earthquakes. — From these con- 
siderations the authors infer, that when earthquakes produce 
