162 GEOLOGY [CHAP. IX 



Letter 509 scored rocks throughout the more level parts of the United 

 States result from true glacier action, it is a most wonderful 

 conclusion, and you certainly make out a very strong case ; 

 so I suppose I must give up one more cherished belief. But 

 my object in writing is to trespass on your kindness and ask 

 a question, which I daresay I could answer for myself by 

 reading more carefully, as I hope hereafter to do, all your 

 papers ; but I shall feel much more confidence in a brief reply 

 from you. Am I right in supposing that you believe that 

 the glacial periods have always occurred alternately in the 

 northern and southern hemispheres, so that the erratic 

 deposits which I have described in the southern parts of 

 America, and the glacial work in New Zealand, could not 

 have been simultaneous with our Glacial period ? From the 

 glacial deposits occurring all round the northern hemisphere, 

 and from such deposits appearing in S. America to be as 

 recent as in the north, and lastly, from there being some 

 evidence of the former lower descent of glaciers all along the 

 Cordilleras, I inferred that the whole world was at this period 

 cooler. It did not appear to me justifiable without distinct 

 evidence to suppose that the N. and S. glacial deposits 

 belonged to distinct epochs, though it would have been an 

 immense relief to my mind if I could have assumed that this 

 had been the case. Secondly, do you believe that during the 

 Glacial period in one hemisphere the opposite hemisphere 

 actually becomes warmer, or does it merely retain the same 

 temperature as before ? I do not ask these questions out of 

 mere curiosity ; but I have to prepare a new edition of my 

 Origin of Species, and am anxious to say a few words on this 

 subject on your authority. I hope that you will excuse my 

 troubling you. 



Letter 510 To J. Croll. 



Down, Jan. 3ist, 1869. 



To-morrow I will return registered your book, which I 

 have kept so long. I am most sincerely obliged for its loan, 



probable Dates of the Glacial and the Upper Miocene Period") 

 published in the Philosophical Magazine, Vol. XXXV., p. 363, 1868, 

 Vol. XXXVI., pp. 141, 362, 1868. His conclusion was that the advocates 

 of the Iceberg theory had formed "too extravagant notions regarding 

 the potency of floating ice as a striating agent." 



