CHAPTEE XIII. 



THE GEOLOGICAL AND MOSAIC EEVELATIONS. 



ONE of the first thoughts which strikes the mind as it con- 

 templates the foregoing view of the natural history of our 

 planet is, that the developments spoken of could have been 

 accomplished only in periods too vast for human conception. 

 Admitting that the process of unfolding which finally resulted 

 in bringing our globe to its present habitable and mature 

 state, commenced when its materials were all in a state of dif- 

 fused igneous gas, it is utterly beyond the power of man to 

 conceive the period which must thence have elapsed before 

 these materials were so far contracted as to admit of the first 

 superficial granitic incrustation. But after these untold 

 myriads of ages had quietly rolled into the depths of the past, 

 sedimentary materials, which, according to statements of Dr. 

 John Pye Smith, as the results of careful measurements, must 

 have had an aggregate thickness of not less than twenty miles, 

 took place, for the most part quietly, at the bottom of the 

 ocean. These materials, including the remains of plants and 

 animals of now extinct species, and whole races of which were 

 successively brought into being and swept away, were after- 

 ward slowly consolidated into the form of the existing fossillif- 

 erous rocks. 



As to the number of years or centuries which must have 

 elapsed during this mighty operation, we have the means of 



