Correspondence — Rev. E. Sill. 479 



of students. But this may well be in a country largely constituted 

 of altered and ig-neous rocks, and amono; students who have reason 

 to look for evidence of geological history as well in the original 

 conditions and successive changes of rock-material as in the 

 character and position of organic remains. The valuable abstract 

 notices of contemporaneous books and memoirs treating of geology 

 and mineralogy are abundantly and carefully given as heretofore. 



ooiaiaEsiPon^zDEisrcE. 



THE POSSIBILITY OF CHANGES OF LATITUDE. 



Sir, — The question discussed in the article on Changes of Axis in 

 the June Number of this Magazine was, — " The earth being rigid, 

 could a deformation tilt the axis in space, or shift the position of 

 the Poles ? " The answer was that a tilt was impossible and a shift 

 improbable. Mr. Fisher, in the July Number, asks for a discussion 

 of the question — " Assuming that a thin crust surrounds a fluid sub- 

 stratum, could then a deformation shift the crust over the nucleus ? 



An obvious reply is, that if the Earth's rigidity has been proved, 

 the discussion would be fruitless. Mr. Gr. H. Darwin, who has been 

 investigating this point, concludes that the Earth is " enormously 

 stiff" (Proc. Royal Soc, No. 188, 1878). 



However, the nature and consequences of the objection to Dr. 

 Hopkins's demonstration may be noticed. His argument was in 

 effect that if there existed a very large fluid nucleus, since the shell 

 would slide freely over it, the Earth's crust could not oppose to the 

 tilting forces so great a resistance as we find from the amount of 

 Precession that it does oppose. To this it is now answered that if 

 the fluid nucleus be spheroidal and rotating, it would resist the 

 tilting force which produces Precession, and the shell would not 

 slide freely. But then would it not also resist the tilting tendency 

 resulting from a deformation ? If Dr. Hopkins's proof from Pre- 

 cession collapses, does not also the supposition become untenable 

 that a fluid nucleus would render easy a shift of the crust ? The 

 suggestion of a fluid substratum seems to lead to the same dilemma ; 

 either the fluid could resist any shift of the crust, or it could not, 

 and so Dr. Hopkins's disproof remains valid. 



A question prior to all this is, Will a change in Latitudes give the 

 best explanation of the phenomena ? E. Hill. 



St. John's College, Cambridge, Atigtist 22nd. 



GEOLOGICAL TIME. 



Sir, — The great difficulty encountered by the geologist, in re- 

 ducing a section of Geological Time to years, from the want of data, 

 is so well known, that bringing the following before your readers 

 may be pardoned, as an attempt to measure a small section of time. 



In the parish of Beith, North Aryshire, the Lower Carboniferous 

 Limestone is extensively wrought as a surface stratum. In some 

 quarries the limestone is preserved by a thick covering of Boulder- 

 clay, and here the surface is ice-polished and finely striated, retaining 



