54 
THE GEOLOGIST. 
earth's crust ; the greater quantity occur in rock-forming minerals, in 
chemical combination with each othei", forming compounds of a higher 
order. Those binary compounds which are met with in quantity as 
minerals, are water, silicic acid as silica, sesqui-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, boracic acid, sesqui-oxide of manganese, 
fluoride of calcium, and bisulphide of iron, as the minerals corundum, 
peinclase, sassoline, braunite, flu or, and pyrite. The first occurs in a 
small rock-formation, as emerj ; the three following are rare and un- 
important ; but the two latter are rock-forming miner-als, 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 magyiesia, in combination 
with silicic, carbonic, and sulphuric acids, form an important portion 
of the earth's crust : the combinations of baryta with the acids ai'e 
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, Jluoride of calcium, 
and bi-sulphide 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. — 
