60 
ELEMENTS OF GEOLOGY. 
CHAPTER IV. 
COJ^SOLIDATION OF STEATA AND PETEIFICATION OP FOSSILS. 
Chemical and Mechanical Deposits.—Cementing together of Particles.—• 
Hardening by Exposure to Air.—Concretionary Nodules.—Consolidating 
Effects of Pressure.—Mineralization of Organic Remains.—Impressions 
and Casts : how formed.—Fossil Wood.—Goppert’s Experiments.—Pre¬ 
cipitation of Stony Matter most rapid where Putrefaction is going on.— 
Sources of Lime and Silex in Solution. 
Having spoken in the preceding chapters of the characters 
of sedimentary formations, both as dependent on the deposi¬ 
tion of inorganic matter and the distribution of fossils, I may 
next treat of the consolidation of stratified rocks, and the pet¬ 
rification of imbedded organic remains. 
Chemical and Mechanical Deposits. —A distinction has been 
made by geologists between deposits of a mechanical, and 
those of a chemical, origin. By the name mechanical are 
designated beds of mud, sand, or pebbles produced by the 
action of running water, also accumulations of stones and 
scoriae thrown out by a volcano, which have fallen into their 
present place by the force of gravitation. But the matter 
which forms a chemical deposit has not been mechanically 
suspended in water, but in a state of solution until separated 
by chemical action. In this manner carbonate of lime is oc¬ 
casionally precipitated upon the bottom of lakes in a solid 
form, as may be well seen in many parts of Italy, where min¬ 
eral springs abound, and where the calcareous stone, called 
travertin, is deposited. In these springs the lime is usually 
held in solution by an "excess of carbonic acid, or by heat if 
it be a hot spring, until the water, on issuing from the earth, 
cools or loses part of its acid. The calcareous matter then 
falls down in a solid state, in crusting shells, fragments of 
wood and leaves, and binding them together. 
That similar travertin is formed at some points in the bed 
of the sea where calcareous springs issue can not be doubt¬ 
ed, but as a general rule the quantity of lime, according to 
Bischoff, spread through the waters of the ocean is very small, 
the free carbonic acid gas in the same waters being five 
times as much as is necessary to keep the lime in a fluid 
state. Carbonate of lime, therefore, can rarely be precipita¬ 
ted at the bottom of the sea by chemical action alone, but 
