134 TRANSACTIONS. 1906-7. 



phere are constant. It follows therefore that the crust of the 

 earth will not follow the shrinking of the interior. Foldings and 

 wrinkles will be produced, and it is the general conclusion, that 

 this is the principal reason for the uplifts of the surface into moun- 

 tain chains. " 



That earthquakes are due to an adjustment of stresses in the 

 earth's crust is admitted by all investigators, but on the cause of 

 the stresses, there is far from unanimity of opinion. 



Although some earthquakes are due to downfalls, and loc'al 

 ones to volcanic eruptions, yet for the great majority another 

 reason or reasons must be found. Of the latter, the contracting 

 force, already alluded to, is the one first to suggest itself, and has 

 for its support at least great plausibility. It has been combated 

 by able investigators, without however being able wholly or 

 satisfactorily to dispose of it completely. 



Leaving out of consideration the earth as a cooling or contract- 

 ing body, let us picture to ourselves the earth at any time in a 

 state of perfect equilibrium, there being no stresses on its surface 

 nor in the crust. Let us note the physical features, the heights 

 of the mountains, the faulting and folding of the rock formations, 

 the depths of the ocean and the distribution of land and water. 

 Now let the atmospheric influences come into play — rain and 

 snow, heat and cold — together with the varying atmospheric 

 pressure. The pre-existing equilibrium will be immediately 

 disturbed; the water, as ripples, creeks, rivers and streams will 

 begin its work of erosion and denudation; heat and frost will 

 assist in the disintegration of mountain masses, and the ocean 

 beds adjoining the continents will be loaded by enormous amounts 

 of detritus carried from the land. Unless there is a continuous 

 and simultaneous adjustment of the change of pressure, the stresses 

 set up will be cumulative and continue so until they exceed the 

 limit of elasticity, when rupture must take place to restore equili- 

 brium for the time being. Rupture would necessarily be accom- 

 panied by earthquakes. 



It is obvious therefore that meteoric or atmospheric influences 

 are capable of setting up stresses on. the earth's surface. It is 

 safe to say that the whole surface of the earth is in a constant 

 tremor due to stresses. But besides the general condition, there 

 are other factors that come into play, and localize in a measure 

 the seismic disturbance. These are mountain masses and ocean 

 depths, especially if they are contiguous. 



