212 T. MeUarcl Reade—A Cooling and Shrinking Globe, 



Effects of Local Shrinkage through Denudation. 



If instead of 4 miles mean thickness of sediment being laid down 

 on the given area of 2000 by 1000 miles, we assume that the same 

 mass has been removed from an equal area by denudation, and, 

 supposing the crust to be free to shrink and could sustain its own 

 weight, the area would be surrounded by trenches respectively 

 2 miles and 1 mile wide at the new surface. It necessarily follows 

 that, even if the shell of compression were to follow the shrink- 

 ing area up at the same rate, there would still remain cavities 

 or trenches below the surface. Secular contraction it is evident 

 could not be operative in producing compression in such a case. 

 Local shrinkage of the crust such as we have assumed does not, 

 however, act in this way. The weight of the crust itself squeezes 

 up all vacuities, and the area adjusts itself to its decrease of volume 

 by normal-faulting and keying up in a wedge-like manner and by 

 compressive extension. 



We have seen that the shell of contraction due to secular cooling 

 is a real and important factor in the earth's economy. This general 

 shrinkage decreases in rate in areas of sedimentation and increases 

 in areas of denudation. It may perhaps be said that if these areas 

 adjoin one will destroy the effect of the other. Such, however, is not 

 the case, as I have shown that the expansion is internal shading off 

 to zero at the boundaries. In the theory of the origin of the earth's 

 folds by secular contraction it is assumed that the folded area is 

 compressed from the outside ; on such a supposition it is a perfectly 

 legitimate inference that the shrinking area would be followed up 

 by the closing in of the shell of compression, and one effect would 

 destroy the other. 



I further maintain that it is a physical impossibility for a layer 

 of the earth 5 miles deep, really but a film iipon the surface of the 

 globe, not only to shear upon itself but to convey thrust through 

 1000 miles of strata in an effective manner. Even if we were to 

 assume that this shell could shear upon itself and accumulate thrust 

 in one place, the compression would be confined to a short distance 

 from the boundary of the area and be inoperative in the centre 

 where mountain ranges are usually thrown up. 



The compression produced by expansion is internal and pro- 

 portional to the depth of the sediment. 



This is an important rule to bear in mind, for it largely determines 

 the axial position of the range. The mere upheaval of some of the 

 strata in anticlinal form will determine where the lateral pressure 

 from the surrounding sheet of strata shall be used up. When one 

 fold is compressed another follows up against it, and so the work of 

 mountain building proceeds. From the comparatively great depth 

 at which the internal expansion acts, underlying rocks are forced up 

 into gneissic and granitic cores, and these expanding laterally by 

 gravitation force back the folded strata and compress them still 

 more, forming in many cases what is known as fan -structure. The 

 very movement of these heated masses of rock forced up from below 



