CHAP. IX GEOLOGICAL CLIMATES 201 



blocks of Silurian and Old Red sandstone rocks which 

 Professor Hull believes could only have been carried by 

 fioating ice. Similar breccias occur in the south of Scotland, 

 and these are stated to be " overlain by a deposit of glacial 

 age, so similar to the breccia below as to be with difficulty 

 distinguished from it." ^ 



These numerous physical indications of ice-action over 

 a considerable area during the same geological period, 

 coinciding with just such a poverty of organic remains as 

 might be produced by a very cold climate, are very import- 

 ant, and seem clearly to indicate that at this remote 

 period geographical conditions were such as to bring about 

 a glacial epoch, or perhaps only local glaciation, in our part 

 of the world. 



Boulder-beds also occur in the Carboniferous formation, 

 both in Scotland, on the continent of Europe, and in North 

 America ; and Professor Dawson considers that he has 

 detected true glacial deposits of the same age in Nova 

 Scotia. Boulder-beds also occur in the Silurian rocks of 

 Scotland and North America, and according to Professor 

 Dawson, even in the Huronian, older than our Cambrian. 

 None of these indications are however so satisfactory as 

 those of Permian age, Avhere we have the very kind of 

 evidence we looked for in vain throughout the whole of 

 the Tertiary and Secondary periods. Its presence in 

 several localities in such ancient rocks as the Permian is 

 not only most important as indicating a glacial epoch of 

 some kind in Palaeozoic times, but confirms us in the validity 

 of our conclusion, that the total absence of any such evidence 

 throughout the Tertiary and Secondary epochs demon- 

 strates the absence of recurring glacial epochs in the 

 northern hemisphere, notwithstanding the frequent recur- 

 rence of periods of high excentricity. 



Warm Arctic Climates in Early Secondary and Palceozoic 

 Times. — The evidence we have already adduced of the mild 

 climates prevailing in the Arctic regions throughout the 

 Miocene, Eocene, and Cretaceous periods is supplemented 

 by a considerable body of facts relating to still earlier 

 epochs. 



^ Geological Magazir.e, 1873, i\ 320. 



