12 THE CHAIN OF LIFE. 



the average been greater in former geological periods than at 

 present, we may perhaps estimate fifty or sixty millions of 

 years as the time required for t^e accumulation of all our 

 formations. Some geologists object to this as too little, but in 

 this some of thev* are influenced by the exigencies of theories 

 of evolution, and others appear to have no adequate conception 

 of the vast lapse of time represented by such numbers, in its 

 relation to the actual rates of denudation and deposition. 



If now we attempt to divide this time among the formations 

 known to us, according to their relative thicknesses, we have, 

 according to an elaborate estimate of Professor Dana, the time 

 ratios of 12, 3, and i for the Palaeozoic, Mesozoic, and Cainozoic 

 periods respectively. Taking the whole time since the begin- 

 ning of the Cambrian as forty-eight millions of years, we should 

 thus have for the Palaeozoic thirtv-six millions, for the Mesozoic 

 nine, and for the Tertiary three. Another calculation, recently 

 made by Professors Hull and Haughton, gives the following 

 ratios : — 



Azoic 34 "3 per cent. 



l^alaeozoic 425 ,, 



Mesozoic and Cainozoic . 23 "a ,, 



This calculation is, however, based on the absolute thickness of 

 the several series as ascertained in Great Britain, without re- 

 ference to the nature of the beds, as indicating different rates 

 of accumulation. Under either estimate it will be seen that 

 the Palaeozoic time greatly exceeds the Mesozoic and Caino- 

 zoic together, and consequently that changes of life seem to 

 have proceeded at an accelerated rate as time wore on. 



Another inquiry of some importance relates to the manner 

 of preservation of fossils, and the extent to which they consti- 

 tute the material of rocks. This inquiry is doubly important, 

 as it bears on the genuineness of fossil remains, and on the 

 means we have of understanding their nature. 



Some rocks are entirely made up of matter that once was 



