52 H. I. JENSEN. 



It will be noticed from the foregoing that according to 

 my contention many volcanoes are situated in those abnor- 

 mal regions in which heating of strata, expansion and 

 elevation are in progress, and in which earthquakes may 

 occur at sunspot maxima when most heat is received from 

 the sun. As these regions coincide with lines of weakness 

 in the earth's crust it is apparent that auy influence which 

 affects the earth as a whole, such as shrinkage of the outer 

 crust, the zone of compression, from loss of heat in the zone 

 of contraction, must have greatest disturbing effect in 

 these places. Hence although seismic and volcanic dis- 

 turbances may occur here at sunspot maxima from local 

 expansion, greatest volcanic and seismic intensity will be 

 experienced at sunspot minima when the earth as a whole 

 is affected. 



IV. Volcanoes. — Professor Judd, in Chapters x., xi. and 

 xii., of his work entitled "Volcanoes," gives an account 

 of what we learn about the earth's interior from volcanoes 

 and of the various theories advanced to account for volcanic 

 action. These theories may be summarised as follows : — 



(1) That of David Forbes and Dana in favour of a liquid 



earth nucleus. 



(2) That of a potentially liquid nucleus. 



(3) That of local development of sufficient heat to produce 



volcanic outbursts by the shearing resulting from the 

 tangential strain between the zone of contraction and 

 the zone of compression. 



(4) That of Humphry Davy attributing volcanic energy 



to the oxidation of deep seated rocks. 



(5) The theory that volcanic action is produced by the 



combination of various gases occluded in deep-seated 

 minerals, and liberated by degrees, as the cooling of 

 the earth proceeds. 



