THE UNSTABLE LAND. 103 



of short, sudden vibrations which travel through the earth at 

 the rate of one or two thousand feet a second. The rate of 

 transmission varies with the intensity of the shock and the 

 nature of the rock materials. When mines of powder were ex- 

 ploded near Holyhead, in Wales, the waves of disturbance 

 were propagated through wet sand at the rate of 951 feet a 

 second ; through friable granite 1,283 feet, and through com- 

 pact granite 1,640 feet a second. Mr. Mallet calculated that 

 during the earthquake of Calabria in 1857, the waves 

 traveled through the earth at the rate of 820 feet a second. 



It appears thus, that the transmission of the waves of 

 disturbance is favored by the solidity of the medium. Hence 

 we discover the explanation of a fact observed two thousand 

 years ago by the Greeks and Romans, that caverns, wells, and 

 quarries retard the progress of the disturbance and thus pro- 

 tect edifices built in their vicinity. 



The surface movement of earthquake waves is radially 

 from a center. The cause of the disturbance must be re- 

 garded as acting with greatest violence at the center, while 

 the effects gradually die out, as the distance from the center 

 increases. But the distances to which the effects are trans- 

 mitted are not equal in different directions ; and this fact is, 

 undoubtedly, attributable to the unequal distribution of the 

 rocks. Generally, the disturbance should be farther felt in 

 the direction of the strike of strata, than in a direction, 

 across the strata; since in the latter direction, the waves have 

 to cross all the interruptions which characterize the strati- 

 fied condition. So, if on one side of an earthquake center, 

 the country is granitic, and in the opposite, is underlaid by 

 Tertiary clays and sands, the granitic region will be most 

 widely and most severely shaken. It is not supposable that 

 the actual center of an earthquake disturbance is at the sur- 

 face. It must exist at some considerable depth beneath the 

 surface. Careful study of the directions indicated by the effects 

 produced, have led not only to the determination of a radial 

 progress over the surface, but to a center of disturbance, in 

 each case, some miles beneath the surface. According to 



