100 Geological Cluing es from Altmatluns of the 



dence of want of homogeneity on the earth's surface to bring a change 

 of axis of rotation within the limits of possibility. 



It is always gratifying to find mathematicians so far interested in 

 our science as to. occupy themselves with the solution of problems, 

 which, when we consider their important bearing, scarcely seem to 

 occupy the attention they would appear to deserve. The early con- 

 dition of our planet is one of these. By carefully considering the 

 possible and probable conditions connected \\ith that state, we dis- 

 miss or retain, as the case may be, much that is of great importance 

 in theoretical geology. Hence the value of such communications as 

 this before us, wherein the conditions for a possible change in the 

 earth's axis are considered. As you are familiar with the reasoning 

 founded on (he figure of the earth, it is merely necessary to remind 

 you of its bearing upon the original fluidity of our planet, a fluidity 

 which theie has been a difficulty in referring to any other cause than 

 to a heat sufiicient to keep the component particles asunder, in such 

 a manner that even to the centre of the mass the pressure was insuf- 

 ficient to prevent a free motion of the particles of matter. 



Sir John Lubbock would appear to have adopted the idea of a 

 cooling body, but referring to the want of homogeneity observed 

 among the parts of the earth thrust up into the atmosphere, and 

 known to us, he calls attention to the effects which might follow this 

 want of homogeneity in our globe. It hence becomes important to 

 learn the value which can be attrihutnd to such a cause. The depth 

 to which we may limit that portion of our spheroid, which is formed 

 of such substances as we find composing masses of rock exposed to 

 our examination, is necessarily very difficult to fix. The highest 

 mountains, j'ising even in the warmest regions of our globe so far 

 into the atmtsphere as to feel the influences of the low temperature 

 surrounding our planet, however vast they may appear to us, merely 

 give a few miles of thickness ; and when we fairly estimate the real 

 depth of the various ascertained accumulations of diff'erent geological 

 ages, we still arrive at such an insignificant portion of the earth's 

 radius, as to see how very little of the component parts of its mass 

 can be known to us. Still we are bound to examine the evidence as 

 to the diffei-ences which may exist as regards homogeneity in the 

 rock masses. Some years since (fifteen), having occasion to estimate 

 the probable specific gravity of fifty miles in depth of the earth's 

 crust,* we found, from direct experiment upon such rocks as appeared 

 important, tliat these varied from 2'49 (chalk) to 3-03 (diallage 

 rock from the Lizard, Cornwall). Upon estimating the masses, 

 taking the surface into consideration, and, therefore, probably giving 

 more differences to the depth supposed, fifty miles, than should be 

 allowed, the mean specific gravity came out as 2-59 higher than the 



* Ivpsearrhes in TliPorPtioal fipology. 



