HIS GEOLOGICAL RELATIONS. 121 



It may be true ttat some geologists, overstepping 

 the limits of legitimate deduction, have attempted to 

 assign absolute dates to certain prehistoric events, but 

 such cases are exceptional, and not generally homo- 

 logated by the cultivators of the science. Did peat- 

 mosses always increase at the same rate, and river-silts 

 and cave-earths accumulate at the same ratio, the age 

 of any imbedded relic would be a matter of the simplest 

 calculation ; but as we have no fixed rate of iacrement, 

 the most pains-takiag investigation in matters of this 

 kind can only be received as a sort of approximation. 

 No doubt, where the rate of progress has been ascer- 

 tained not for one year but for a series of years, the 

 approximation to dates must become very close, and 

 such calculations are as much entitled to acceptance 

 as life-statistics or other well-ascertained averages ; 

 but in the meantime geological observations have been 

 neither sufiiciently long nor sufficiently accurately 

 made, and all that can be safely done is to abide by 

 a relative chronology. We may not be able to assign 

 to any event a date in years and centuries, but we can 

 say, judging from the known operations of nature 

 whether it could have possibly taken place withiu the 

 lapse of six or of sixteen centuries, and this, however 

 little, is always some approach to a satisfactory con- 

 clusion. If we cannot give facts, we can at least correct 

 assumptions, and next to the value of positive know- 



