FOSSIL BOTANY. 235 



or layers which were the first in order in the formation of 

 the present crust of the earth, but few traces of plants are 

 disceruable. The interpretation of these facts is clear, 

 when we remember that geology teaches that our earth 

 was, at a very remote period of time, a molten mass or 

 incandescent globe, revolving around the sun as now; in 

 the course of time, by the gradual loss of a portion of 

 heat, a superficial crust was formed, upon which, as soon 

 as conditions were favorable, appeared first plants, and 

 then animals. 



199. With continued loss of heat from the earth, and 

 other climatal changes not easily explained, the kinds of 

 plants and animals also gradually but very slowly changed, 

 and after a great lapse of time these were seen to be re- 

 placed by others suited to the changed environment ; and 

 such continuous modification has proceeded to this day. 

 But the forces now operating in producing changes of the 

 present surface of the earth were likewise acting — and, of 

 course, with similar results — during all this immense period 

 of time. Therefore, denudation of elevated portions of the 

 earth's surface, and deposition of the transported material, 

 caused a succession of layers of mud to be formed. Occa- 

 sionally remains of plants and animals then living, espe- 

 cially the harder parts in their structure, became encased 

 in this mud. These layers of mud afterwards became 

 converted into rock, in the layers of which the remains 

 of plants and animals, or their imprints (fossils), are 

 contained, and afibrd to day the record — as yet but par- 

 tially examined — whose reading gives the past or geologi- 

 cal history of our earth. For convenience of study, this 

 period of developement is divided into Times, called 

 (beginning with the earliest) Archaean (Gr. archein, to 



