266 GEOLOGY. 



that time we find in the northern portions of the* earth 

 the remains of such plants as would naturally flourish in 

 warm climates. There are palm-like leaves and fruits, 

 leguminous seeds of arboreal growth, twigs and leaves 

 of mimosa, laurel, and other plants, all allied to what 

 now grow in southern latitudes, and quite in contrast 

 with the plants now found in the same localities. Asso- 

 ciated with the twigs, leaves, and fruits found in the 

 strata are beds of lignite, or brown coal, indicating a 

 great abundance in the vegetation. From the high lati- 

 tude to which warm weather extended in that period, 

 Agassiz remarks that the Tertiary age may be called the 

 geological summer. 



374. Strata of Diatoms. There are found here and 

 there in the Tertiary system strata which are made up 

 of what were formerly supposed to be infusorial animals. 

 The diatoms, however, which compose by far the largest 

 proportion of the rock, are now ascertained to be vege- 

 table, and not animal. I have already noticed these 

 plants, and the strata which they have formed, in 240, 

 and will not dwell on them here. As these diatomacea? 

 are so exceedingly minute, a very long age must have 

 been required for the accumulation of thick beds of silex 

 from their remains, such as are found about Richmond, 

 in this country, and in Bilin, in Bohemia. As it takes 

 187 millions of them to make a single grain, it could have 

 been only by the contributions of countless generations 

 of them that a stratum of from 14 to 30 feet in thickness 

 was laid down. 



375. Tertiary Animals. So far to the north was the 

 climate warm in the Tertiary age, that as it was with 

 plants, as stated in 373, so it was with animals such 

 kinds of animals as now flourish in tropical climates were 

 then in high northern latitudes, crocodiles, turtles, gigan- 

 tic sharks, pachydermatous quadrupeds, etc. The earli- 

 est remains of birds are found in the strata of the Eo- 

 cene period, the dawn of the Tertiary. So also serpents 



