T. Mettard Reade — Secular Straining of the Earth. 345 



from a central source. I have also in the work referred to indicated 

 how this supply of lava may, through local variations of temperature 

 induced by sedimentation, be pumped up to the surface, there to 

 undergo the modifications exhibited in ordinary volcanic phenomena. 



The explanations therein attempted of deep volcanic phenomena 

 do not, however, exclude the co-existence of a secular and wider 

 cause, such as that formulated by Dr. Johnston-Lavis. 



I have very little faith in the efficacy of tangential compression, 

 induced by secular cooling, as a mountain-building agent, as the 

 shell-of-compression is too thin to produce results of the magnitude 

 we see in nature. 



The shell-of-contraction is, as I have shown, 1 vastly greater than 

 the shell-of-compression, and under certain conditions might be a 

 very efficient machine for the forcing up of lava to the surface. It 

 is these necessary conditions that I propose to discuss, and to inquire 

 whether there is a probability of their existence. 



Firstly, then, we must ascertain under what conditions the secular 

 contraction takes place. The shell-of-compression is, according to 

 the highest estimate, not more than five miles thick, and according to 

 Mr. Fisher is under two miles. 



Below the under-surface of this shell the whole of the crust of the 

 earth, to a depth at which cooling ceases, is in a state of contraction. 

 This shell of contraction, for the proper conception of what takes 

 place, may be divided into an infinite number of shells, each of 

 which contracts at a different rate, the shell of greatest contraction 

 being situated at a maximum depth of 54 miles according to Fisher. 2 



It will thus be seen that it is not a simple case of contraction, like 

 that of the tire of a wheel or the hooping of a gun, but an infinite 

 compound series of tires all contracting at different rates. Assuming 

 that the shells are homogeneous and free from fractures, it would be 

 very difficult to say what the effective contractile force of this 

 compound would amount to, even if we knew the tensile strength 

 of the materials composing it, which we do not. 



It could not contract as a whole without internal movements in 

 itself partially destroying its effectiveness for compressing the 

 nucleus. The problem is still further complicated by the com- 

 pressive-extension produced by the gravitation of the overlying shell 

 of compression as well as of the various shells or matter constituting 

 itself. The idea may be best realized by stretching an elastic band 

 on a plane surface and then weighting it ; the weighting will inter- 

 fere with the contractile force the band would otherwise develop, 

 and if sufficient destroy its movement. I have shown that at the 

 zone of greatest contraction " practically tension could not take place, 

 as the superincumbent strata would by vertical compression elon- 

 gate the rocks at the zone of greatest contraction to fill the vacuities 

 that otherwise would be created." s At what depth compressive- 

 extension would take the place of tensile stress I am not prepared to 



1 Origin of Mountain Ranges, chap. xi. 



2 Physics of the Earth's Crust, second edition, p. 106. 



3 Origin of Mountain Rauges, p. 125. 



