THE HISTORY OF LIFE 183 



hard and fast lines in their distribution, since they 

 often pass from one stratum into another. 



THE FIRST BOOK OF THE STONE BIBLE. 



In the classification of the various strata the lowest 

 main division the first great book is called by geo- 

 logists, Primary Eoclcs. Underlying these are still older 

 layers or strata called by geologists, the pre-Cambrian 

 .Rocks. These strata are often highly metamorphosed, 

 and may even have been converted into igneous rocks ; 

 this may have been done by the melting of the rocks, 

 which were usually laid down by the action of water. 



" In these primeval deposits there are records of 

 denudation and deposition, of alternate sedimentation 

 and terrestrial movements, of stupendous and prolonged 

 volcanic activity, and of distinct though scanty proofs 

 that plant and animal life had already appeared 

 upon the face of the globe." 1 Here we find evidences 

 of great volcanic action. " The vast scale of these 

 volcanic eruptions may be inferred from the fact that 

 in the Lake Superior region the accumulated materials 

 discharged at the surface attained a thickness which 

 has been estimated at more than six and a half 

 miles." 2 . . . " Hills and valleys, lines of cliff and 

 crag, rocky slopes and undulating hollows have been 

 revealed by the slow denudation of the pre-Cambrian 

 strata." 3 In fact the physical conditions of the earth 

 at this vastly remote period were similar to those 

 of the present day. 



i (' Text-book of Geology," Sir Archibald Geikie, F.R.S., 3rd 

 edition, 1893, p. 681.) 



Idem, p. 692. 3 Idem, p. 693. 



