42 H. I. JENSEN. 



and solar heats, and variations in gravitational influences. 

 As due to secondary causes, lie mentions the following 

 classes of earthquakes : — 



1. Those due to faulting. 



2. Those due to explosions of steam. 



3. Those due to volcanic evisceration. 



4. Those due to chemical degradation. 



5. Those due to effect of oceanic tides. 



6. Those due to variation in barometric pressure. 



7. Those due to fluctuations in temperature. 



8. Those due to rain and snow. 



1. Loss of telluric heat leads to secular contraction, and 

 this latter process brings about normal faulting. In regions 

 where radial exceeds circumferential contraction it brings 

 about overthrust faulting as well. Reverse and overthrust 

 faults are, however, more frequently due to the expansion, 

 folding and contortion of strata, resulting from rise of 

 isogeotherms in heavily sedimented areas. Both varieties 

 of faulting may be said to be primarily due to the constant 

 exchange of heat between the hot core of the earth and its 

 outer shell, and again between the earth's crust and space. 



Since it has been shown probable (see my previous paper 

 and Part II of this), that the earth receives more heat from 

 the sun in sunspot maximum periods than in minimum 

 periods, it is apparent that sunspot effect must vary in 

 accordance with the geological nature of particular regions. 

 When most heat is received from the sun radiation into 

 space is checked, and secular contraction is delayed. At 

 the same time, in regions undergoing heating from within, 

 consequent upon the rise of isogeotherms, the thermo- 

 dynamic processes might be accentuated. 



In areas like Japan, Java and Argentina, where many 

 districts are undergoing slow elevation, summer earth- 

 quakes are more frequent than winter ones, for the reason 





