342 Mr. C. Chree on some Applications of 



almost infinite variety of colour of allotropic silver, whilst 

 normal salts of silver when formed with colourless acids are 

 mostly colourless. On the other hand, the greater activity 

 of allotropic silver and its less specific gravity would seem to 

 indicate a simpler molecular constitution than that of normal 

 silver. 



XLIII, Some Applications of Physics and Mathematics to 

 Geology. By 0. Chree, M.A., Fellow of King's College, 

 Cambridge. 



[Continued from p. 252.] 



Part II. Some Geological Theories. 



rpHE belief that the present spheroidal form of the earth 

 X necessarily betokens a previous liquid, or at least plastic, 

 condition seems amongst Geologists almost as universal as 

 the belief that the earth but for the development of rotation 

 must have been a spherical body. Whether this latter con- 

 clusion has any satisfactory basis apart from philosophical 

 speculations, it is not my present object to inquire. But 

 supposing, for the sake of argument, that the natural form of 

 the earth as undisturbed by rotation is spherical, the con- 

 clusion that it ever was in a liquid or even in a plastic state 

 throughout is, according to the preceding results, not established 

 by its present spheroidal form. Yet even in such a standard 

 work as Geikie's ' Text-book of Geology,' after reading the 

 discussion on p. 12 and the footnote attached, I fail to detect 

 a trace of the idea that the polar flattening might be called 

 forth by rotation in a truly solid body. 



Various geological writers, it is true, speak of a solid earth 

 as capable of changing its form, but they seem in reality to 

 regard the change as due to rupture or to the development of 

 a plastic condition. This appears, for instance, to be the view 

 actually held by Mr. Herbert Spencer in a short paper* 

 entitled " The Form of the Earth no proof of Original 

 Fluidity." This paper has been referred to with a somewhat 

 inaccurate conception of its value and results by two recent 

 geological writers, so it claims some notice at our hands. 

 The first of the two writers referred to, Mr. W. B. Taylor f, 

 says : — " It is now nearly forty years since Herbert Spencer, 

 with a juste r physical insight [than Sir W. Thomson and 

 Professor Tait], contended and satisfactorily showed that a 

 solid earth (of any shape) would assume the oblate spheroidal 



* Phil. Mag. [3] vol. xxx. 1847, pp. 194-196. 



t American Journal of Science, vol. xxx. 1885, pp. 258, 259. 



