32 SUBSTANCES OF WHICH BOCKS ABE COMPOSED.

known earth combined with carbonic acid,in which state it forms lime-stone, marble,and chalk : these only differ from each otherby different degrees of hardness, or of cry-stallization. Mountains composed of limeare denominated calcareous. When lime isunited with sulphuric acid, it forms the stonecalled gypsum.

Magnesia has rarely been found pure in anative state. It enters into the compositionof some rocks, to which it generally communi-cates a soapy feel, a striated or striped tex-ture, and sometimes a greenish colour.

Iron appears to be more abundant thanmagnesian earth ; it forms a constituent partof numerous rocks and stones; to it theymost frequently owe their colour ; the earthswhen pure are white. Iron when in combina-tion with the earths is like them an oxyd, orametal united w ith oxygen. To the presenceof iron, the increase of specific gravity inmost stones may also be attributed, if it muchexceed S’oO, or they are heavier than waterin a proportion exceeding that of five to two,or c 2| to 1. From these five substances united

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