92 THE UNIVERSE. 



SO extremely small that it has been calculated it Avould re- 

 quire about 10,000,000 to make a pound of chalk, and that 

 there are more than 150,000,000 in a cubic metre (cube 

 of 39'37100 inches). Favoured by then' inconceivable 

 fecundity, these animalcules filled up the cretaceous seas, 

 and by accumulating in layers at their bottoms their 

 skeletons have formed the capacious chalk strata of which, 

 at the present time, some mountains are composed. Some- 

 times 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 microscoj)e. Sometimes the weight 

 of the new superimposed layers has reduced those at the 

 base to a fine powder, and then Ave find only a soft thin 

 clay. 



To sum up then : 



The layers of oiu- calcareous moimtains 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 Avhich is only made tip of microscopic 

 shells. 



The formation of the first surprises, that of the latter 

 confounds, us. 



