454 Notices of Memoirs — Professor Sollaiss Address 



a thickness of 34 miles of sediment. How mucli greater, then,, 

 must have been the interval required for the elaboration of the 

 whole organic world ! . . . . 



Geologic Periods of Time. 



Before proceeding to the discussion of estimates of time drawn 

 from a study of stratified rocks, let us first consider those which 

 have been already suggested by other data. These are as follows : — 

 (1) Time which has elapsed since the separation of the earth and 

 moon, fifty-six millions of years, minimum estimate by Professor 

 G, H. Darwin. (2) Since the " consistentior status," twenty to 

 forty millions (Lord Kelvin). (3) Since the condensation of the 

 oceans, eighty to ninety millions, maximum estimate by Professor 

 J. Joly. 



It may be at once observed that these estimates, although 

 independent, are all of the same order of magnitude, and so far 

 confirmatory of each other. Nor are they opposed to conclusions 

 drawn from a study of stratified rooks ; tlaus Sir Archibald Geikie, 

 in his Address to this Section last yeai", affirmed that, so far as these 

 were concerned, 100 millions of years might suffice for their 

 formation. There is, then, very little to quarrel about, and our 

 task is reduced to an attempt, by a little stretching and a little 

 paring, to bring these various estimates into closer harmony. . . . 



A review of the facts before us seems to render some reduction in 

 Dr. Joly's estimate imperative. A precise assessment is impossible, 

 but I should be inclined myself to take oJBf some ten or thirty 

 millions of years. 



We may next take the evidence of the stratified rocks. Their 

 total maximum thickness is, as we have seen, 265,000 feet, and 

 consequently, if they accumulated at the rate of one foot in 

 a century, as evidence seems to suggest, more than twenty-six 

 millions of years must have elapsed during their formation. 



Obscure Chapter in the Earth's History. 

 Before discussing the validity of the argument on which this last 

 result depends, let us consider how far it harmonizes with previous 

 ones. It is consistent with Lord Kelvin's and Professor Darwin's, 

 but how does it accord with Professor Joly's? Supposing we reduce 

 his estimate to fifty-five millions : what was the earth doing during 

 the interval between the period of fifty-five millions of years aga 

 and that of only 26|^ millions ago, when, it is presumed, sedimentary 

 rocks commenced to be formed? Hitherto we have been able to 

 reason on probabilities ; now we enter the dreary region of 

 possibilities, and open that obscure chapter in the history of the 

 earth previously hinted at. For there are many possible answers to 

 this question. In the first place, the evidence of the stratified rocks 

 may have been wrongly interpreted, and two or three times the 

 amount of time we have demanded may have been consumed in 

 their formation. This is a very obvious possibility, yet again our 

 estimate concerning these rocks may be correct, but we may have 

 erroneously omitted to take into account certain portions of th© 



