IxXXvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



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

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

 possible and probable conditions connected with that state, we dismiss 

 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 the figure of the earth, it is merely necessary to remind 

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

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

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

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

 sufficient 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 ho- 

 mogeneity in our globe. It hence becomes important to learn the value 

 which can be attributed 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 exami- 

 nation, is necessarily very difficult to fix. The highest mountains, 

 rising even in the warmest regions of our globe so far into the atmo- 

 sphere 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 different 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 

 differences 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 im- 

 portant, that 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 differ- 

 ences to the depth supposed, fifty miles, than should be allowed, the 

 mean specific gravity came out as 2 "59, higher than the density of 

 2*.5, that commonly adopted, and yet sufficiently near that density for 

 the purpose intended. 



Laplace estimated the mean density of our planet as 1*5.5, the 

 solid surface being considered as 1, hence taking the interior density 

 higher than that of the external parts. We see, looking at such mi- 

 neral substances as form masses of rock, that they are all oxides, but 

 of the depth to which these oxides may descend we know nothing. 

 Unless we suppose them oxides from the beginning, that is, from 

 the time the matter of our earth may have been gathered together as 

 a body revolving around the sun, an hypothesis for which it would 

 * Researches in Theoretical Geolosv. 



