~ Parr Il, §i.] CHEMICAL CONSTITUTION OF CRUST. 59 
- dioxide, CO, present in the air, in rain, in the sea, and in ordinary 
terrestrial waters. This oxide is soluble in water,! giving rise then 
to a dibasic acid termed Carbonic Acid (Kohlensiure) CO (OH), or 
H, CO., which, in combination with calcium, has been instrumental 
in the formation of vast masses of solid rock. Carbon dioxide 
constitutes a fifth part of the weight of ordinary limestone. 
Sulphur (Soufre, Schwefel) $8, occurs uncombined in occasional 
deposits like those of Sicily and Naples, to be afterwards described, 
also in union with iron and other metals as sulphides; but its 
principal condition as a rock-builder is in combination with oxygen 
as sulphuric acid (Schwefelsaure) H, 50, which with lime forms beds 
of sulphate. 
Calcium enters into the composition of many crystalline rocks 
in combination with silica and with other silicates. But its most 
abundant form is in union with carbon dioxide when it appears as the 
mineral calcite (Ca CO,) or the rock limestone. Calcium carbonate, 
being soluble in water containing carbonic acid, is one of the most 
universally diffused mineral ingredients of natural waters. It 
supplies the varied tribes of mollusca, corals, and many other 
invertebrates with the mineral substance for the secretion of their 
tests and skeletons. Such too has been its office from remote 
geological periods, as is shown by the vast masses of organically 
formed limestone which enter so conspicuously into the structure of 
the continents. In combination with sulphuric acid, calcium forms 
Important beds of gypsum and anhydrite. 
Magnesium, Potassium, and Sodium play a less conspicuous 
but still essential part in the composition of the earth’s crust. 
Magnesium in combination with silica forms a class of silicates of — 
prime importance in the composition of voleanic and metamorphic 
rocks. As a carbonate it unites with calcium carbonate to form 
the widely diffused rock, dolomite. Potassium or Sodium combined 
with silica is present in small quantity in most silicates. In union 
with chlorine as common salt sodium is the most important mineral 
ingredient of sea-water, and can be detected in minute quantities in 
air, rain, and in terrestrial waters. In the old chemical formule 
hitherto employed in mineralogy the metals of the alkalies and 
alkaline earths are represented as oxides. Thus lime (calcium 
monoxide), soda (sodium monoxide), potash (potassium monoxide), 
magnesia (magnesium oxide), are denoted as in union with carbonic 
acid, sulphuric acid, silica, &c., forming carbonates, sulphates, silicates 
of lime, soda, &e. 
Tron and Manganese are the two most common heavy metals, 
occurring both in the form of ores and as constituents of rocks. Iron 
is the great pigment of nature, Its peroxide, sesquioxide, or 
ferric oxide forms large mineral masses, and together with the 
protoxide or ferrous oxide occurs in smaller or larger proportions in 
* One volume of water at 0° C. dissolves 1-7967 volumes of carbon dioxide; at 
15° C. the amount is reduced to 1:0020 volumes. 
