CAUSES OF CLIMATAL CHANGE 393 



might occur under conditions less exaggerated. Sir Charles, 

 like all other thoughtful geologists, was well aware of the gen- 

 eral fixity of the areas of the continents, though with great 

 modifications in the matter of submergences and of land con- 

 ditions. The union, indeed, of these two great principles of 

 fixity and diversity of the continents lies at the foundation of 

 theoretical geology. 



We can now more precisely indicate this than was possible 

 when Lyell produced his "Principles," and can reproduce the 

 conditions of our continents in even the more ancient periods 

 of their history. An example of this may be given from the 

 American continent, which is more simple in its arrangements 

 than the double continent of Eurasia. Take, for instance, the 

 early Devonian or Erian period, in which the magnificent flora 

 of that age, the earliest certainly known to us, made its appear- 

 ance. Imagine the whole interior plain of North America 

 submerged, so that the continent is reduced to two strips on 

 the east and west, connected by a belt of Laurentian land on 

 the north. In the great mediterranean sea thus produced, 

 the tepid water of the equatorial current was circulated, and it 

 swarmed with corals, of which we know no less than 150 species, 

 and with other forms of life appropriate to warm seas. On the 

 islands and coasts of this sea was introduced the Erian flora, 

 appearing first in the north, and with that vitality and colonizing 

 power of which, as Hooker has well shown, the Scandinavian 

 flora is the best modern type, spreading itself to the south. A 

 very similar distribution of land and water in the Cretaceous 

 age gave a warm and equable climate in those portions of North 

 America not submerged, and coincided with the appearance of 

 the multitude of broad-leaved trees of modern types which ap- 

 peared in the middle Cretaceous, and prepared the way for 

 the mammalian life of the Eocene. 



We have in America ancient periods of cold as well as of 

 warmth. I have elsewhere referred to the boulder conglomer- 



