CLASSIFICATION OF MINERALS. 73 



gredients of granite, mica slates, volcanic rocks, shales, 

 sandstones and various soils. No element is therefore more 

 important than this in the constitution of the earth's strata . 

 and it is specially fitted for this preeminence by its superioi 

 hardness, a character it oommunicates to the rocks in which 

 it prevails. N.ext to silica, rank lime and carbon ; for carbon 

 with oxygen constitutes carbonic acid, and this combined 

 with lime, produces carbonate of lime, the ingredient which, 

 when occurring in extended beds, we call limestone and 

 marble Again, lime combined with sulphur and oxygen, 

 (sulphuric acid,) makes sulphate of lime, or common gypsum. 

 Iron is very generally diffused ; it is one of the constituents 

 of many siliceous minerals, and forms vast beds of ore. 

 Oxygen, as has been implied, is a constituent in all the rocks 

 above mentioned, and besides, is an essential part of the 

 atmosphere and water ; it is the most universally diffused of 

 the elements. It is united with hydrogen in the constitu- 

 tion of water, and with nitrogen in the constitution of the 

 atmosphere. Chlorine combined with sodium constitutes 

 common salt, which occurs in sea water and brine springs, 

 and is also found in vast beds in some rock strata. 



It is thus seen how few are the elements essential to the 

 framework of our globe. The various metallic ores, of less 

 general diffusion, are however of vast economical importance 

 to man, and multiply considerably the number of mineral 

 species. Those important to the general student, however, 

 are comparatively few. The whole number of well estab- 

 lished species in the mineral kingdom is about 600 ; of these, 

 more than two-thirds are known only to the mineralogist. 



It is the province of chemistry to discuss fully the nature 

 of the elements, and their modes of combination. It is suf- 

 ficient to add here, for the benefit of any who may not have 

 the requisite elementary chemical knowledge, how the chem- 

 ical names of minerals indicate their composition. Terms 

 such as oxyd of iron, chlorid of iron, express a combination 

 of iron with the element oxygen, or chlorine ; so also sul- 

 phuret of iron is a compound of iron with sulphur. The 

 force of the terminations id or uret is always as here ex- 

 plained. Protoxyd and peroxyd imply different proportions 



Which are the next most common ingredients of rocks ? Mention 

 the other ingredients alluded to. What ie an oxyd ? a chlorid? a aul- 

 phuret 1 a carbonate ? 



