TREE ANCESTORS 



rence, and many students believe that we are today living in a 

 fourth interglacial period. This would account for the present 

 climate with which man is familiar, a climate anomalous when 

 compared with that of the Tertiary or the Cretaceous, since the 

 glacial periods partially due to the extension of land masses and 

 elevation are the result of the consequent changes in the atmos- 

 pheric and oceanic circulation. 



Thus a second principle may be set forth, namely, that through- 

 out the bulk of geologic time climates were more uniform than they 

 are at present. The truth of this statement will be abundantly 

 shown when the former distribution of our forest types is passed 

 in revdew, although the actual amount of change has usually been 

 overestimated. 



Geologists in dealing with the rocks of the earth's crust and their 

 succession in time divide them into geological formations which 

 are thus the units of geological history and each formation may be 

 considered as a page .of earth history or a phase of geological time. 

 From the remains of animals and plants that were preserved in 

 the rocks of a geological formation, i.e., its fossil content, and from 

 the nature of the enclosing rock that constitutes the matrix of the 

 fossils, i.e., their physical and mineralogical character, and from their 

 relations to adjacent rocks, it is possible to in a measure restore 

 the environment which existed at the time that they were deposited, 

 namely the topography, temperature, rainfall, forestation, etc. 



These stratigraphic units or formations representing phases of 

 time are grouped together in larger units known as stages (etages) 

 representing what are called ages of time and these in turn are 

 grouped into units of still larger magnitude known as epochs of 

 time or series of stages, as for example the Eocene, Oligocene and 

 Miocene series of stages or epochs of time. These series or epochs 

 are further grouped into systems of series (rocks) or periods of 

 time, as for example, the Carboniferous system or coal period, the 

 Cretaceous system or chalk period, and so on. The Eocene, Oli- 

 gocene, Miocene and Pliocene series collectively constitute the Ter- 

 tiary period during which both the Mammals or warm blooded 

 non-avian am'mals and the Angiosperms or flowering plants under- 



