74 THE UNIVERSE. 



sea, and by forming large calcareous mountains with their 

 debris; in fact, when we examine the layers of which 

 these are composed, we perceive that they are formed 

 entirely of polypoids and bivalves which swarmed in the 

 ancient oceans of the globe. 



Ground to dust by the furious waves, these creatures 

 have only here and there left a few traces to attest their 

 presence, and serve as a light to the modern investigators 

 of science. 



Such is the opinion of Lyell and most modern geo- 

 logists. In support of this view it has been observed that 

 certain lagoons are filled with a calcareous mud, evidently 

 due to the detritus of j^olypoids, and that, so soon as this 

 is dried, it exactly resembles the chalk of om- ancient 

 mountains. 



To the action of the waves, the chief agent in trans- 

 forming polypoids and bivalves into calcareous strata, there 

 is joined another, much less energetic it is true, but 

 extremely curious. An observing man of genius, Mr. 

 Darwin, relates that all round the madrepore islands the 

 transparency of the water allows one to see shoals of fish, 

 principally of the genus Sparus, which feed on the tips 

 of the branched polypoids, exactly as flocks of sheep 

 brouse on the pastui-age of our meadows. In order to 

 nourish themselves with the workman, they devour along 

 with him certain parts of his edifice; and as these are 

 absolutely indigestible, the result is, according to the 

 English savant, that a part of the chalky substance which 

 encumbers the bottom of the sea in the vicinity of the 

 madrepore reefs, comes from the defecations. When the 

 Spari are dissected, their alimentary canal is seen filled 

 with pure chalk. 



The madrepore islands generally lie on an upheaval of 



