CORRESPONDENCE 275 



I have lately propounded another simple theory explaining the cause of the 

 various phenomena of comets' tails other than by an enormous force of 

 repulsion residing in the sun ; and apart from the proofs of my theory, have 

 shown that the behaviour of certain comets establishes, beyond doubt, the 

 truth of J. H. J cans' s recent tidal theory of the formation of the solar system 

 {Journal of the Torquay Nat. Hist. Soc, 1920). 



This, like Drayson's theory, is an apt illustration of a remark of Herschel's, 

 in his Discourse on Natural Philosophy, that any occurrence which, according 

 to theory, should not happen (i.e. which appears paradoxical) often serves as a 

 clue to new discoveries. 



Now that I have established the independence of geologists, permitting 

 orthodox astronomers to retire from the controversy, which has become more 

 purely geological, I have a few remarks to make in reply to the letter of Mr. 

 Tyrrell. It is rather typical of the confusion of ideas regarding the Glacial 

 Epoch. The one controlling factor for ice periods having been disallowed 

 by astronomers, geologists have hitherto run wild, assimilating ideas, now 

 from one theorist and then from another, in fruitless attempts to pierce the 

 mists surrounding this question, while they fail to recognise facts lying at 

 their very feet. 



Owing to this tangle of opinions I purposely confined myself to bringing 

 to light the great fact of a geologically recent, I might say historic, ^ glaciation, 

 and though I have definite ideas about other glaciations in the post-tertiary 

 period and about other glacial epochs, I am not prepared to be cross-examined 

 on all points, say such as the cause of the Permian glacial epoch, or the 

 presence of coal at Spitzbergen, as if my credit was imperilled by my 

 inability to answer. I might as well be asked to describe why mammoth are 

 found in such vast numbers in Siberia and remains of hippopotamus in great 

 quantities in some caves of Southern Europe, which is probably due to a glacia- 

 tion, but is not directly material to the main question of a recent glaciation. 

 I have not space to fully traverse every statement made, but Mr. Tyrrell 

 speaks as if " crustal unrest " was the precursor of glacial epochs. On the 

 contrary, it is the immense weight of piled-up ice which depresses the crust 

 in one place and elevates it in another. This subject has been fully studied 

 in North America, and accounts for many of the raised beaches round our 

 coast. That is the real reason why crustal unrest is associated in time with 

 glacial periods. I admit, however, that but for Greenland our northern 

 glaciations would have been much less severe. It is difficult to imagine a 

 continent nearly as large as Europe without Russia, covered all over with 

 mountains as high as the Bernese Oberland : this condition must be almost 

 unique in the world's history. Its gradual uplift (it is now falling, so far as 

 is known) would account for the severity of glaciations working up to a maxi- 

 mum, but these matters can best be left for discussion in the years to come. 

 Until we establish the last and recent glaciation as a working theory, we shall 

 never make progress. 



Nor are we yet in a position to object to the " regularity of astronomical 

 cycles" which, moreover, has not been established, nor does it necessarily 

 follow. Slight differences in the centre of gravity of the earth might in 

 time greatly affect the centre of polar movement,^ and therefore the intensity 



1 According to Aristotle, the Chaldeans had records of the conditions of 

 the seasons of this period, which were distinguished by alternating "ekpy- 

 rosis " and " kataclysmos." 



2 If the true centre were to be 6° nearer than the ecliptic pole, the O.E. 

 would only attain 23° at its maximum, instead of 35° as now. Again, there 

 is thought to be a probability, from a gyrodynamic point of view, of this 

 movement being a spiral decreasing to nothing of obliquity, and then in- 

 creasing (Crabtree). 



