GEOLOGICAL PRINCIPLES 7 



went their most obvious differentiation and distribution over the 

 face of the earth. 



These systems of rocks representing the history of periods of 

 time are themselves gathered together into groups that represent 

 what are called eras of time, thus the Triassic, Jurassic and Creta- 

 ceous periods constitute the Mesozoic group of periods or the 

 Mesozoic era of time — the era of gymnosperms (more familiarly 

 conifers) among plants and the era of reptiles among animals. 



All this may seem comphcated and abtruse to the non-geological 

 reader, but it must be obvious that we cannot discuss the noble 

 races of trees that have passed across the face of nature in past 

 ages without a definite chronology and nomenclature, any more 

 than we could live our daily lives and transact its affairs comfort- 

 ably and effectively without time pieces and calendars. Nor can 

 we compare the contents of the rocks of different countries and de- 

 termine the place of origin or the subsequent history of the migra- 

 tions and extinctions of plants or animals without a carefully 

 worked out chronology. Without such our situation might be 

 compared to that in civil life before standard time was adopted. 



The following somewhat abbreviated table will furnish the reader 

 with the geological divisions that it will be necessary to use in 

 trying to picture the history of the ancestors of our trees. It also 

 gives a few of the facts of earth history and organic history that 

 will serve to emphasize and fix the former in mind. 



To those who notice the fate of the leaves each autumn it may 

 seem that for any adequate geological record of bygone vegetation 

 the conditions for the successful preservation or fossilization of 

 parts of plants must have been very different in the past from 

 what they are at the present time. Except for the fact that the 

 continental masses are somewhat larger now than was the case 

 for the greater part of geological time this is not so. The facilities 

 for fossilization in inland and upland regions have always been 

 limited to lake and flood-plain (river overflow) deposits. Moun- 

 tain lakes have frequently preserved marvellous records of the 

 contemporaneous Kfe as that of the Miocene lake of Florissant in the 

 heart of the Colorado Rockies or the even more celebrated Miocene 



