1886.] NEW YORK ACADEMY OF SCIEJTCES 33 



laid the foundation of a solid structure, subsequently built up 

 to the surface. Because, first, there is no probability that the 

 superficial crust had a higher specific gravity than the underly- 

 ing fluid ; and, second, if its gravity were greater, and when 

 broken up its fragments sank, they must soon have been melted 

 by the greater heat below ; and when, by this process, the outer 

 zone of the earth had acquired a pasty consistence, its cohesion 

 could not have been overcome by sheets of crust, even if a little 

 denser. Hence, a cool, external crust, a hot, viscous zone, and 

 a hotter fluid nucleus seem logical necessities. 



PROXIMATE CAUSES OF EARTHQUAKES. 



Atmospheric Condiiions. — If it is true, as claimed on the pre- 

 ceding pages, that earthquakes are the vibrations attending the 

 folding and breaking of rocks which have been in a state of 

 strain, it is evident that the provoking cause of any special par- 

 oxysm might be a comparatively trifling affair — some feather 

 that should break the camel's back. Thus, Ave have reason to 

 believe that atmospheric conditions may jirecipitate these catas- 

 trophes. The pressure of the atmosphere on the earth's surface 

 is 14.7 pounds to the squnre inch — that is, a little over 2,000 

 pounds to the square foot, or about 30,000,000 tons to the square 

 mile. Now, it sometimes happens that the mercury oscillates 

 two inches in the tube of a barometer in connection with some 

 violent storm; and it is true that the areas of low and high 

 pressure change position quite rapidly. Hence, if it should hap- 

 pen that the underlying rocks were from lateral pressure in a 

 state of strain that had nearly reached the limit of resistance, a 

 change of atmospheric pressure equivalent to two, or even one 

 inch of mercury (equal to 1,000,000 or 2,000,000 tons per mile), 

 might be the cause of a rupture. So, the popular belief, that 

 peculiar atmospheric conditions have had an influence in caus- 

 ing earthquakes, is not so absurd as it might seem. 



Accumulation of Sediments. — Another cause which has cer- 

 tainly operated to disturb the static equilibrium of the earth's 

 crust, is the transfer of the products of erosion from the land to 

 the bottom of an adjacent sea basin. Over all land areas where 

 the rainfall is considerable, there is a constant wearing away of 

 the surface by chemical and mechanical agents. About one- 

 fourth of the material removed is dissolved and may be carried 

 to the opposite side of the earth before it is precipitated ; but 

 the other three-fourths, in the form of gravel, sand and clay, are 

 simply held in suspension by running waters and are deposited 

 as soon as their motion is arrested. Rivers, rivulets, and shore 

 waves are constantly engaged in transporting material from the 

 land to the deeper water bordering the coasts; there spreading it 



