54 



THE GEOLOGIST. 



earth's crust ; the greater quantity occur in rock-forming minerals, iu 

 chemical combination with each other, forming compounds of a higher 

 order. Those binary compounds whicli are met with in quantity as 

 minerals, are zmfer, silicic acid as silica, sesqu i-oxide of iron as hematite, 

 and chloride of sodium as rock-salt. Those substances will be referred 

 to more fully hereafter, in considering them as minerals. We 

 also find alumina, magnesia, horacic acid, sesqui-oxide of manganese, 

 Jiuoride of calcium, and bisulphide of iron, as the minerals corundum, 

 periclase, sassoline, braunite, fluor, and pyrite. The first occurs in a 

 small rock-formation, as emery ; the three following are rare and un- 

 important ; but the two latter are rock-forming minerals, though not 

 very abundant. 



It remains for us now to consider the combinations of these binary 

 compounds among themselves. Water plays an important part in 

 combination, forming an essential and considerable constituent of 

 many rock-forming minerals, as gypsum, chlorite, and serpentine. 

 The first three earths, alumina, lime, and magnesia, in combination 

 with silicic, carbonic, aiid sulphui'ic acids, form an important portion 

 of the earth's crust : the combinations of baryta with the acids are 

 comparatively trifling. The first two alcalies — ■potash and soda — also 

 occur abundantly in wide-spread minerals ; lithia is much rarer. 



The first three acids are also of the greatest importance ; particu- 

 larly silicic acid, which, in a free state, as quartz, or in chemical 

 combination with the earths and alcalies, it is estimated, alone con- 

 stitutes forty- five per cent, of the earth's mineral crust. Carbonic acid, 

 although far inferior to the last-named acid, also enters as a main 

 constituent of wide-spread mountain-masses. Sulphuric acid is also 

 an important rock-constituent : boracic acid is unimportant as to 

 quantity. 



The oxides of iron and manganese — the protoxides of which only 

 exist in combination — form an essential constituent of many wide- 

 spread rock-forming minerals. Chloride of sodium, jiuoride of calcium, 

 and bisulphide of iron, are not regarded as entering into combinations. 



XV. The general conclusions we arrive at regarding the chemistry 

 of the rock-forming minerals is this : with the exception of the nine 

 following minerals, which are either elements or binary com- 

 pounds, viz. — 



