MINERALS, 181 



Will be found in most treatises upon chemistry; suffice it that a 

 union of silicmm and oxygen forms nearly one half the solid part 

 of the earth's crust, being a chief ingredient in all the principal 

 rocks. It appears nearly pure in the state of transparent rock 

 crystal, and is the chief ingredient of our ordinary flints and 

 sands. Aluminium in combination with oxygen, is the base of 

 the various clays and clayey slates. Potassium in combination 

 with oxygen, and also sodium with oxygen, constitute very im- 

 portant ingredients of the rocks and earths under the well known 

 forms of potash and soda, and the latter in combination with 

 chlorine forms muriate of soda or common salt, so. widely preva- 

 lent in the ocean, and in beds of rock salt. Magnesium i^. the 

 base of manganese a chief ingredient of the chalks, and magne- 

 sias limestones; and calcium is the metallic base of lime, and is 

 found in abundance in the various limestones and gypsnms. Be- 

 sides the simple substances named, we have the metallic miner* 

 als, which are usually found in beds or veins. The most of these 

 are so well known that they are recognised at a glance. Iron is 

 the most useful and most abundant, when combined with sulphur 

 it crystalises in cubes, is of a bright yellow, and is often mista- 

 ken for gold; this variety is termed iron pyrites. Iron is likewise 

 found in combination with oxygen and carbon, and occasionally 

 nearly pure. Lead is a well known metal occurring principally 

 in union with sulphur, under the form called galena, in cubic 

 crystals. Copper is found in combination with oxygen and sul- 

 phur, and also native, or pure ; in combination with carbon and 

 oxygen it assumes beautiful tints of blue and green. Tin, zinc, 

 and manganese are too well known to need any particular descrip- 

 tion here. It is said that tin is only found in the primitive or 

 lowest order of rocks, and that the tin mines of Comwall^xtend 

 many hundred feet under the sea, and that the noise of the waves 

 and the rolling of the pebbles can be distinctly heard. We need 

 not describe the precious metals, silver, gold, and platina, as 

 everybody is familiar with their general properties. The various 

 stibstances which form the crust of the earth, and which have 

 been investigated by the persevering energy of man, are arranged 

 into two great classes, which embrace all the various soils, sands, 



