^2^5 



times, different races of plants and animals lived, whose 

 specific forms changed with those successive times, and 

 being preserved to us, are characteristic or indicative of 

 these times. 



It also follows that the chemical processes which are 

 necessarily coincident with the phenomena of life, — the 

 decomposition of the carbonic acid of the atmosphere by- 

 respiration of plants, — the decomposition of sea water by 

 marine plants and marine animals,— the secretion of solid 

 carbonate of lime and pure silica in the substance of shells, 

 corals, and sponges, must have continued through times and 

 under conditions equal to produce the amount of those effects 

 which Geology has measured and made known. 



These things are generally known and admitted ; but it is 

 possible by scrutinizing more minutely the texture of rocks, 

 to extend the data and fortify the reasoning. Our admira- 

 tion of the degree in which the hard parts of animals have 

 contributed to augment the so-called mineral masses of the 

 globe will indeed rise to astonishment^ if we call to our aid 

 the magical power of the achromatic microscope, and in- 

 vestigate with it the ultimate constitution of the limestones, 

 flints, sands, and clays, in which the unassisted eye sees 

 nothing but a mere aggregate of earthy particles. 



We may effect this in at least four ways ; — 1st, by cutting 

 stones into thin slices, so that they become transparent ; 

 2ndly, by incineration ; 3rdly, by pulverizing and washing 

 softer substances, such as chalk ; and 4thly, by distilling 

 chemically some parts of a rock, and leaving others for 

 examination in a finely divided state. 



This investigation has drawn forth a crowd of observers, 

 of whom Ehrenberg is the chief. It results from their in- 

 quiries, that in the superficial deposits of Geology, which 

 are scarcely counted among the old stratified masses, but 

 belong to comparatively recent periods, vast numbers of 



