Chalk and Flint Formation. 19 



whose fall in recent times has been witnessed and 

 universally recognized. 



It is necessary, however, to point out that some 

 unwarranted conclusions have been drawn by men of 

 science from fchis nebular theory. For it has been 

 supposed that the heat of the nebulous materials was 

 such as would now suffice to sublimate all things into 

 vapour; that, when this vapour contracted into the 

 planetary form, it assumed the condition of an 

 incandescent fluid globe; and Sir William Thomson 

 has expended time and pains in calculating how 

 many millions of years it must have taken for the 

 earth to cool down to its present state, from that 

 condition of fiery fluid incandescence — a calculation 

 which may rather serve to shake the credit of that 

 assumed condition of the earth. It cannot be proved 

 that either twenty millions or four hundred millions 

 of years ago, 2 there was more heat, more electricity, 

 or more light in the materials of the solar system 

 than there is now. " There are," says Sir Charles 

 Lyell, 3 " no positive proofs of a secular decrease of 

 internal heat accompanied by contraction. On the 

 contrary, Laplace has shown, by reference to 

 astronomical observations made in the time of 

 Hipparchus, that in the last two thousand years at 

 least there has been no sensible contraction of the 

 globe by cooling ; for, had this been the case even to 

 an extremely small amount, the day would have been 

 shortened ; whereas its length has certainly not 

 diminished during that period by -g-^-th part of 



2 These are the minimum and maximum limits assigned by Sir 

 William Thomson for the time necessary for the earth to cool down 

 to its present temperature from fluid incandescence. Trans. Royal 

 Society of Edin., xxiii. 157. 



3 " Principles of Geology," vol. i. p. 304. 



c 2 



