TKA^^ERS. — On the former Waruith in hiijJi Xurtherii Latitudes. 471 



He further alleges, that m the latter case the effect of the trade winds 

 would be to impel the heated currents of the Atlantic and Pacific Oceans 

 further towards the North Pole, thereby causing the ice within the arctic 

 regions to disappear, and so producing a climate sufficiently mild to admit 

 of the existence in those regions of a fauna and flora as rich and varied as 

 that which characterized them during any past geological period. 



In the nineteenth chapter of his work he gives a diagram showing the 

 eccentricity of the earth's orbit for 3,000,000 of years in the past, and 

 1,000,000 of years to come, at periods of 50,000 years apart, and tables 

 showing those matters more in detail at intervals of 10,000 years. On 

 looking over the diagram it will be seeii that there were three principal 

 periods in the past during which the eccentricity rose to a high value. It 

 is to one or other of the last two that Mr. Croll assigns the geological period 

 familiarty known as the glacial epoch, preferring, however, the later to the 

 earlier one for reasons into which he enters at some length. 



The later period consisted of two separate maxima, the earlier one being 

 the greater of the two, and separated from the latter extreme by an interval 

 during which the eccentricity was considerably diminished although still 

 comx3aratively high, the whole period occupying 160,000 years, one-half of 

 which would represent the united length of the cold johases in each hemis- 

 phere. Now, assuming Mr. Croll's views to be correct, the glacial epoch 

 which was brought about by this condition of things would continuously 

 affect each hemisphere alternately for a period of about 10,500 years, owing 

 to the precession of the equinoxes. But the operation must have been com- 

 plicated by the circumstance that, during the 160,000 years in question, there 

 was a period of lesser eccentricity occupying some 80,000 years, during which 

 the intensity of the cold would have been diminished in each hemisphere. 

 It will be observed therefore that, during the period referred to, the con- 

 ditions requisite, according to Mr. CroU's theory, for the production and 

 removal of glacial conditions in the northern hemisphere would have 

 occurred with varying intensity at alternating intervals of 10,600 years 

 about four times, owing to the precession of the equinoxes, or, in other 

 words, that the arctic circle would, at the intervals referred to, alternately 

 have enjoj-^ed atmospheric conditions which would admit of the existence of 

 a luxuriant fauna and flora, or such as were utterly opposed to the existence 

 of any terrestrial life. 



But a glance at Mr. CroU's diagram will, I think, sufficiently show that, 

 whilst his theory may abundantly account for an extension of pre-existino- 

 glaciated conditions in each hemisphere during periods of maximum eccen- 

 tricity, there is nothing in it to lead to the conclusion that the polar ice has 

 ever been completely removed since its first formation in mass, which, as I 



