140 RELATIVE GEOLOGICAL ANTIQUITY OF TREES. 



of several hundred species of plants, all of them extinct, and 

 all resembling plants of warmer climates. 



During these geological periods, the marine or amphibian 

 reptiles of the Secondary rocks were replaced by numerous 

 mammalia of enormous size. This era has therefore been 

 called by Agassiz, the " Age of Mammals." 



If we would form for ourselves an idea approximating to a 

 true conception of the appearance which the earth presented 

 during this era, we must consider the land as still partially 

 insular, or a loosely-connected group of islands, covered with 

 low volcanoes. The problem, what is it ? To unite more and 

 more the solid land and to restrain the oceanic life. Through 

 the influence of air and light shall the life on the land be in- 

 creased, and the life developed by the waters be circumscribed ; 

 and through the influence of a diversity of climate, a greater 

 variety in creation shall be produced, and thus the regular 

 powers of Nature be brought to work together harmoniously. 



The climate of the northern hemisphere, which had been, 

 during the Tertiary epoch, considerably warmer than now, so 

 as to allow the growth of palm trees in our present temperate 

 zones, became much colder toward the end of that period, in 

 consequence of the gradual accumulation of snow and ice at 

 the poles. The ice formation began manifestly about this 

 time, for as we go further back into the history of the earth, 

 through the Tertiary deposits to the time of the Chalk forma- 

 tion, we find neither erratic blocks, nor any indications of the 

 marks of glaciers ; on the contrary, the proofs become more 

 and more apparent, that the earth in these earlier epochs was 

 much warmer than it is at present. Hence the appearance of 

 ice at the poles and on' the summits of mountains must be re- 

 garded as comparatively speaking a recent event in the history 

 of creation, and is contemporaneous with some of the last 

 revolutions to which the earth has been subjected. 



The glacial or ice period is considered to have produced 

 those numerous detached fragments of foreign rocks which 

 we find scattered over the surface of the soil, and which are 

 known under the name of erratics, boulders, or grey-heads. 

 These drift materials are called Diluvium, as their transporta- 



