376 Mr. J. Croll on Geological Time, and the probable 



Stratified formations 10,000 feet in thickness, for example, 

 may, under some conditions, have been formed in as many 

 years, while under other conditions it may have required as 

 many centuries. Nothing whatever can be safely inferred as to 

 the absolute length of a period from the thickness of the stra- 

 tified formations belonging to that period. Neither will this 

 method give us a trustworthy estimate of the relative lengths of 

 geological periods. Suppose we find the average thickness 

 of the Cambrian rocks to be 26,000 feet, the Silurian to be 

 28,000 feet, the Devonian to be 6000 feet, and the Tertiary to 

 be 10,000 feet, it would not be safe to assume, as is sometimes 

 done, that the relative duration of those periods must have cor- 

 responded to these numbers. Were we sure that we had got 

 the correct average thickness of all the rocks belonging to each 

 of those formations, we might probably be able to arrive at the 

 relative lengths of those periods ; but we can never be sure of 

 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 sea and land during the Cambrian, Silurian, Old-Red- 

 Sandstone, Carboniferous, and other early periods of the earth's 

 history differed more from the present relative positions than 

 the relative positions of sea and land during the Tertiary period 

 differed 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. Of course it is by no means asserted 

 that they are thicker than we 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 palseontological 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 

 a 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 palseontological method is 

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



