GEOLOGICAL SURVEY OF THE TERRITORIES. 337 



tertiary. In this category of older deposits, with a more tropical flora, I 

 would place the Green Eiver tertiary beds, those of Mississippi, studied 

 by Lesquereux, and those of Brandon, Vermont. 



In the miocene age the continental surface was broader, the lake 

 basins of the West contained only fresh water, and the land surface 

 was covered with vegetation very much like that of the present day, a 

 number of miocene species still existing. The climate of the continent 

 in the miocene age was much milder than now. Fan-palms then grew 

 as far north as the Yellowstone Eiver, and a flora flourished in Alaska 

 and on Greenland as varied and as luxuriant as now grows along the 

 fortieth parallel. At this time there must have been some sort of land 

 connection between our continent and Europe on the one hand and Asia 

 on the other. The flora of all these regions was essentially the same, 

 and a large number of plants were common to the three continents. In 

 this age the mammalian fauna of our continent exhibited the same re- 

 markable development that it did in Europe and Asia ; and over our 

 western plains roved herds of great quadrupeds, rivaling in number 

 and variety those that have struck with wonder and surprise every 

 traveler in South Africa. 



This state of things seems to have continued through the pliocene 

 age and up to the time when the climate of the continent was completely 

 revolutionized by the advent of the " ice period." The change which 

 took place at that time was such as taxes the imagination to conceive 

 of as much as it taxes the reasoning powers to account for. 



We have seen that in the middle tertiary age the climate of Alaska 

 and Greenland was that of New York and St. Louis at present. In the 

 next succeeding period, the glacial epoch, the present climate of Green- 

 land was brought down to New York, and all the northern portion of 

 the continent wrapped in ice and snow. This change was undoubtedly 

 gradual, (for nature does not often " turn a corner,") but it is plain that 

 it must have resulted in the gradual driving southward of all the varied 

 forms of animal and vegetable life that were spread over the continent to 

 the Arctic Sea. When glaciers reached as far south as the fortieth parallel 

 it is evident that a cold, temperate climate prevailed in Mexico, and that 

 only in the south of Mexico would the average annual temperature have 

 been what it was previously in the latitude of New York. We must con- 

 clude, therefore, that the herds of mammals which once covered the plains 

 of the interior of North America were forced by the advancing cold into 

 such narrow limits in Southern Mexico that nearly all were exterminated. 

 Plants bore their expatriation better ; inasmuch as a tree, even of the 

 most gigantic size, will live upon the space occupied by its roots, pro- 

 vided the climatic conditions are favorable ; while one of the larger 

 mammals would require at least a thousand times this space for its sup- 

 port. As a consequence, we find the present flora of our continent much 

 more like that of the miocene than is our fauna, though the change to 

 which I have referred seems to have been fatal to quite a number of the 

 most abundant and interesting of our miocene forest trees. Of these, 

 the Glyptostrobus may be taken as an example. This was a beautiful 

 conifer which, in miocene times, grew all over our continent and over 

 Northern Europe. In the change to the glacial period, however, it was 

 exterminated, both there and here, yet continued to exist in China — 

 where a miocene colony from America had taken root — and it is growing 

 there at the present time. This great ice- wedge which came down from 

 the north separated very widely many elements in our miocene flora 

 which have never since been reunited, so that when the storm had 

 passed and better days had come, and the Mississippi Valley and Atlan- 

 22 g 



