ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 47 



Also (as explained p. 53), so far as heat is lost by means of 

 vulcanism the resultant earth contraction does not give an effect 

 in crustal corrugations in addition to that due to the transfer of 

 the magma. 



But in estimating the effect of secular cooling upon corruga- 

 tion, it must be borne in mind that the earth is so large that cor- 

 rugations may have begun on one part of the crust, while other 

 parts were still subject to tension, as explained, page 46. Under 

 these circumstances corrugations might be produced which 

 would be compensated by tensile cracks elsewhere. Such ten- 

 sile cracks may become filled with sediment, with vein material, 

 or with igneous injections. In so far as such compensated cor- 

 rugations have been produced during the early history of the 

 earth, these deformations are in excess of the amount which it is 

 allowable to attribute to secular cooling. Davison suggests that 

 early in the history of the earth the continental masses might 

 have passed earlier from the stage of tension to the stage of 

 compression than the sea beds, and that a part of the crustal 

 flexures of the continents were, therefore, compensated by ten- 

 sile fractures under the sea. 1 



It appears from the foregoing that the corrugations of the 

 earth due to secular cooling follow from the difference in the 

 loss of heat by conduction and by convection, and that developed 

 by earth movements ; and from an irregular distribution of the 

 resultant stresses, which in some places may be tensile, and at 

 the same time in other places be compressive. Ordinarily the 

 loss of heat by conduction only has been considered. It is clear 

 that in secular cooling we have an important, but probably by 

 no means an adequate, cause to account for observed crustal 

 deformation. 



Vulcanism. — The second cause to which I shall appeal to 

 explain shell corrugations is vulcanism. 



At the outset it should be said that the quantity of igneous 

 material which now reaches the surface of the earth is no crite- 

 rion by which to judge past extrusions, for at times of regional 



1 Loc. cit., p. 241. 



