346 T. Hellard Reach — Secular Straining of the Earth. 



say, as the molecular condition of the materials of the earth's crust 

 will be modified by the pressure tending to produce compressive- 

 extension. 



In addition to this natural difficulty arising from the conditions 

 being outside any laboratory experiments so far made, is also one 

 arising from our ignorance of the nature and extent of the continuity 

 of strata at great depths. At the surface the rocks are divided by 

 faults and fractures, which must in te re fere very much with their 

 action as an effective contracting and compressing envelope to the 

 earth's nucleus. 



I have now indicated some of the principal dynamical conditions 

 of the problem which it will be necessary to have answered more 

 clearly before we can arrive at anything more than uncertain con- 

 clusions as to the effect of the contracting shell on the earth's nucleus, 

 and I have avoided any appeal to geological phenomena either for or 

 against what may be provisionally designated " the contracting shell 

 theory " of deep volcanic action. It would seem that some compres- 

 sion must be set up in the nucleus by secular contraction, which if not 

 an effective cause in itself of the expulsion of lava, it may be in 

 conjunction with local thermal effects such as I have indicated in 

 chap. xxi. of the " Origin of Mountain Ranges." I do not think it 

 theoretically necessary that there should be actual rifts in the shells 

 at the depths at which release of pressure will allow the earth's 

 ma^ma to change from the solid to the fluid state. Lava if under 

 constant pressure would force or bore its way through any strata 

 exhibiting local weakness without a rift occurring. Indeed, as I 

 have already explained, no rift could occur at great depths, because 

 of the compression produced by gravitation or otherwise compressive- 

 extension. As I have elsewhere attempted to show, the rifts connected 

 with volcanoes must be comparatively speaking surface phenomena, 

 and the " feeders " or communications of the volcanoes with the central 

 reservoir must be necks or pipes, not fractures. 



So much for the earth. If we on the other hand turn to the 

 moon, we find the ring-mountains and other volcanic phenomena 

 studding its surface more readily explicable by the contracting shell 

 theory. As gravity on the moon's surface is only about one-sixth 

 that on the earth's, it is quite likely, as I have already suggested, 

 that the pressure was insufficient to produce solidification of the 

 moon's nucleus, which would therefore be a fluid mass inclosed in 

 a gradually thickening solid shell. 1 If so, we have all the conditions 

 present for effective contraction and expulsion of the fluid magma 

 at the surface, for the shell would probably have a greater coefficient 

 of tensile strength than that of the earth, and its greater thickness 

 proportionally to the moon's diameter would make it a stronger 

 compressing vessel, while the work it had to do, from the smaller 

 size of the nucleus, its fluid condition and low specific gravity, would 

 be much less than in the case of a planet of the magnitude and 



1 See note by me appended to the paper by the Rev. F. Grensted entitled 

 "Theory of the Airless and Waterless Condition of the Moon," Proceedings of the 

 Liverpool Geol. Soc. Session 1887-8. 



