282 Dr. J. Croll on the Cause of 



upon it. In Chap. xvii. of ' Climate and Time ' I have 

 endeavoured to show that although much has been written on 

 the imperfection of geological records, yet the imperfection of 

 those records in regard to past glacial epochs has not received 

 the attention which it really deserves. 



It must be borne in mind, however, that it does not follow 

 from the Physical Theory of Secular changes of Climate, that 

 because the eccentricity may have been high at some parti- 

 cular period there must necessarily have been a glacial epoch. 

 The erroneous nature of this misapprehension of the theory 

 has already been shown at considerable length*. Eccentri- 

 city can produce glaciation only through means of physical 

 agencies, and for the operation of these agencies a certain 

 geographical condition of things is absolutely necessary. We 

 know with certainty that during the Tertiary period the 

 eccentricity was at times exceptionally high, as, for example, 

 2,500,000 and 850,000 years ago ; but whether a glacial epoch 

 occurred at these periods depended, of course, upon whether 

 or not the necessary geographical conditions then obtained. 

 Supposing the necessary geographical conditions for glaciation 

 did exist at the two periods in question, still if these condi- 

 tions differed very much from those which now obtain, the 

 glacial state of things then produced would certainly differ 

 from that of the last glacial epoch. This is obvious, for the 

 same physical agencies acting under very different conditions 

 would not produce the same effects. Under almost any geo- 

 graphical condition of things eccentricity would produce 

 marked effects, but the effects produced might not amount to 

 glaciation. In the Tertiary age, during high eccentricity, the 

 effects resulting might possibly have been as well marked as 

 they were during the glacial epoch ; but these effects must 

 have differed very much from those produced at that epoch. 

 We have seen that, owing to that peculiar geographical condi- 

 tion of things existing during the Tertiary period, the physical 

 agents brought into operation by a high state of eccentricity 

 would have a much greater influence in raising the tempera- 

 ture of the northern hemisphere when the winters occurred 

 in perihelion, than they would have in lowering the tempera- 

 ture of that hemisphere when the winters were in aphelion. 

 At the periods 2,500.000 and 850,000 years ago, for example, 

 those physical agents would no doubt produce quite a tropical 

 condition of climate in high northern latitudes when the 

 winters were in perihelion, while it is quite probable they 

 may not have been able to produce glaciation when the 

 winters were in aphelion. It is more than likely that the 



* Phil. Mag., February 1884 j American Journal of Science, Feb. 1884. 



