394 CAUSES OF CLIMATAL CHANGE 



ates of the Huronian, of the early Lower Silurian, and of the 

 Millstone grit period of the Carboniferous ; but I have not ven- 

 tured to affirm that either of these periods was comparable in 

 its cold with the later glacial age, still less with that imaginary 

 age of continental glaciation, assumed by the more extreme 

 theorists. We know that these ancient conglomerates were 

 produced by floating ice, and this at periods when in areas not 

 very remote, temperate floras and faunas could flourish. The 

 glacial periods of our old continent occurred in times when the 

 surface of the submerged land was opened up to the northern 

 currents drifting over it mud and sand and stones, and render- 

 ing nugatory, in so far, at least, as the bottom of the sea was 

 concerned, the effects of the superficial warm streams. Some 

 of these beds are also peculiar to the eastern margin of the 

 continent, and indicate ice drift along the Atlantic coast much 

 as at present, while conditions of greater warmth existed in the 

 interior. Even in the more recent glacial age, while the moun- 

 tains were covered with snow, and the low lands submerged 

 under a sea laden with ice, there were interior tracts in some- 

 what high latitudes of America in which hardy forest trees and 

 herbaceous plants flourished abundantly, and these were by no 

 means exceptional " interglacial " periods. Thus we can prove 

 that from the remote Huronian period to the Tertiary, the 

 American land occupied the same position as at present, and 

 that its changes were merely changes of relative level, as com- 

 pared with the sea ; but which so influenced the ocean currents 

 as to cause great vicissitudes of climate. 



Uniformitarian geologists have recently been taunted with a 

 willingness to assume great and frequent elevations and sub- 

 mergences of continents, as if this were contrary to their 

 principle. But rational uniformitarianism allows us to use any 

 cause of whose operation in the past there is good geological 

 evidence, and Lyell himself was perfectly aware of this. 



While no geologists can fail to appreciate the evidence of 



