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



as a result of an increase. Until those physical circumstances 

 were discovered it was impossible that the true cause of the 

 glacial epoch could be known. Many of the indirect and phy- 

 sical effects, which in reality were those that brought about 

 the glacial epoch, could not, from the nature of things, have 

 been known previously to recent discoveries in the science of 

 heat. When the excentricity is about its superior limit, the com- 

 bined effect of all those causes to which I allude is to lower to a 

 very great extent the temperature of the hemisphere whose win- 

 ters occur in aphelion, and to raise to nearly as great an extent 

 the temperature of the opposite hemisphere, where winter of 

 course occurs in perihelion. I have made these remarks in order 

 to obviate certain objections to which I shall afterwards have 

 occasion to refer. 



Astronomy and physics not only afford a cause for those ab- 

 normal conditions of climate during geological epochs, but they 

 seem to afford also (at least so far as regards very recent epochs) 

 a probable means of arriving at a pretty accurate determination 

 of the date at which those conditions prevailed. 



On examining the Tables of excentricity given in former pa- 

 pers* for a million of years back, it will be seen that there are 

 two periods of great duration during which the excentricity con- 

 tinued at a high value. The one period extended from about 

 980,000 to about 720,000 years ago, and the other period began 

 about 240,000 years ago and extended down to about 80,000 

 years ago. At first I felt disposed to refer the glacial epoch (the 

 time of the true boulder-clay) to the former period ; and the 

 latter period, I was inclined to believe, must have corresponded 

 to the time of local glaciers towards the close of the glacial epoch, 

 the evidence of which, in the shape of moraines, is to be found 

 in almost every one of our highland glens. 



There was, however, one formidable objection to this view of 

 the matter which presented itself to my mind at the time. I 

 found, from calculations based on the amount of sediment carried 

 into the Gulf of Mexico by the Mississippi River, that the North 

 American continent is being lowered by subaenal denudation at 

 the rate of 1 .} s $ of a foot per annum, and that, consequently, 

 if the rate of denudation be as great in this country as in Ame- 

 rica, which is by no means improbable, then about 500 feet 

 must have been removed off the face of the country and carried 

 by our rivers into the sea since the period of the boulder-clay, if 

 that period is to be placed 700,000 years backf. It would 

 therefore follow that the general features of the country must 

 now be totally different from what they were at the close of the 



* Phil. Mag. for January 1866 and February 1867- 

 t Phil. Mag. for February 1867, p. 130. 



