THEORY of the EARTH. 289 



Therefore, in knowing how thofe animals had lived, or with 

 what they had been fed, we fhall have learned a moft intereft- 

 ing part of the natural hiftory of this earth ; a part which it 

 is necefTary to have afcertained, in order to fee the former ope- 

 rations of the globe, while preparing the materials of the pre- 

 fent land. But, before entering upon this fubjecl, let us exa- 

 mine the other materials of which our land is formed. 



GRAVEL forms a part of thofe materials which compofe our 

 folid land ; but gravel is no other than a collecYion of the frag- 

 ments of folid flones worn round, or having their angular 

 form deflroyed by agitation in water, and the attrition upon 

 each other, or upon fimilar hard bodies. Confequently, in 

 finding mafles of gravel in the compofition of our land, we 

 mufl conclude, that there had exifted a former land, on which 

 there had been tranfacled certain operations of wind and wa- 

 ter, fimilar to thofe which are natural to the globe at prefent, 

 and by which new gravel is continually prepared, as well as old 

 gravel confumed or diminifhed by attrition upon our mores. 



SAND is the material which enters, perhaps in greatefl quan- 

 tity, the compofition of our land. But fand is no other than 

 fmall fragments of hard and folid bodies, worn or rounded 

 more or lefs by attrition ; confequently, the fame natural hifto- 

 ry of the earth, which is inveftigated from the mafles of gravel, 

 is alfo applicable to thofe maffes of fand which we find forming 

 fo large a portion of our prefent land throughout all the earth. 



CLAY is now to be confidered as the laft of thofe materials 

 of which our ftrata are compofed j but, in order to underftand 

 the nature of this ingredient, fomething mufl be premifed. 



CLAY is a mixture of different earths or hard fubftances, 

 in an impalpable ftate. Thofe fubftances are chiefly the filice- 

 ous and aluminous earths. Other earths are occafionally mixed 

 in clays, or perhaps always to be found in fome fmall portion. 

 But this does not affect the general character of clay ; it only 

 forms a fpecial variety in the fubjecl. A fenfible or confidera- 



O o ble 



