84 THE UNIVERSE. 



up the cretaceous seas, and by accumulating in layers at 

 their bottoms their skeletons have formed the great masses 

 of chalk strata of which, at the present time, some moun- 

 tains are composed. Sometimes they are solely constituted 

 of little shells still entire, as is seen in the rocks of Sicily 

 and the chalk of Meudon, when submitted to the micro- 

 scope. Sometimes the weight of the new superimposed lay- 

 ers has reduced those at the base to a fine powder, and then 

 we find only a soft thin clay. 



To sum up, then : 



The layers of our calcareous mountains may be of three 



kinds : One composed of entire shells piled up ; the second 



are formed of shells crushed fine ; and, finally, there are 



some the bulk of which is only made up of microscopic 



shells. 



The formation of the first surprises, that of the latter 



confounds, us. 



