146 Fossils. 



the compound earths and stones, forming 

 the immense mass of the solid nucleus 

 of the globe. It is the base of almost 

 all the scintillating stones, such as flint, 

 rock, crystal, quartz, agate, calcedon, 

 jasper, &JV. The sand of rivers and of 

 the sea-shore, chiefly consists of it. 



Argillaceous earth is found in clay^ 

 schistus, or slate, and in mica. This 

 species of earth is ductile with water, it 

 then hardens and contracts by heat, so 

 as to be of the greatest use in forming 

 brick, or stones of any required form or 

 size. 



Terra ponder osa is generally found in 

 two states, viz. united to vitriolic acid, 

 when it is called calk; or to fixed air, when 

 it is called terra ponderosa aerata. This 

 earth is distinguishable by its great speci- 

 fic gravity, being four times as heavy as 

 water. 



Magnesia is found sometimes pure in 

 nature, but it is generally obtained by art 

 from some of its combinations. It gives 

 a peculiar character to the substances of 

 which it forms a part. The stones which 

 contain magnesia in considerable quantity 

 ' .have generally a smooth and unctuous feel ? 



