76 THE CANADIAN NATURALIST. [March 



this. Those formations all, at one time, formed sea-bottoms; 

 and we can only measure those deposits that are now raised 

 above the sea level. But is it not probable that the relative po- 

 sitions of the sea and land during the Cambrian, Silurian, Old- 

 Red-Sandstone, Carboniferous, and other early periods of the 

 earth's history diifered more from the present relative positions 

 than the relative positions of sea and land during the Tertiary 

 period diifered from the relative positions which obtain at 

 present ? May not the greater portion of the Tertiary deposits 

 be still under the sea-bottom ? And if this be the case, it may 

 yet be found at some day in the distant future, when these 

 deposits are elevated into dry land, that they are much thicker 

 than we now conclude them to be. It is simply asserted that 

 they may be thicker for anything that we know to the contrary ; 

 and the possibility that they may, destroys our confidence in the 

 accuracy of this method of determining the relative lengths of 

 geological periods. 



The palaeontolo leal method of estimating geological time, 

 either absolute or relative, from the rate at which species change 

 appears to be even still more unsatisfactory. If we could ascer- 

 tain by some means or other the time that has elapsed from some 

 given epoch (say, for example, the glacial) till the present day, 

 and were we sure at the same time that species have changed at 

 an uniform rate during all past ages, then, by ascertaining the 

 percentage of change that has taken place since the glacial 

 epoch, we should have a means of making something like a rough 

 estimate of the length of the various periods. But without 

 some such period to start with, the palaeontological method is 

 useless. It will not do to take the historic period as a base-line. 

 It is far too short to be used with safety in determing the dis- 

 tance of periods so remote as those which concern the geologist. 

 But even supposing the palaeontologist had a period of sufficient 

 length measured off correctly to begin with, his results would 

 still be unsatisfactory ; for it is perfectly obvious, that unless the 

 climatic conditions of the globe during the various periods were 

 nearly the same, the rate at which the species change would 

 certainly not be uniform. But we have evidence, geological 

 as well as cosmical, that the climate of our globe has at various 

 periods undergone changes of the most excessive character. 



The palaeontological method, as we have already seen, will 

 give 60 millions of years or 240 millions of years as the period 



