62 
ELEMENTS OF GEOLOGY. 
Traces of impressions and casts are often extremely faint. 
In some loose sands of recent date we meet with shells in so 
advanced a stage of decomposition as to crumble into pow¬ 
der when touched. It is clear that water percolating such 
strata may soon remove the calcareous matter of the shell; 
and unless circumstances cause the carbonate of lime to be 
again deposited, the grains of sand will not be cemented to¬ 
gether ; in which case no memorial of the fossil will remain. 
In what manner silex and carbonate of lime may become 
widely diffused in small quantities through the waters which 
permeate the earth’s crust will be spoken of presently, when 
the petrifaction of fossil bodies is' considered; but I may re¬ 
mark here that such waters are always passing in the case 
of thermal springs from hotter to colder parts of the interior 
of the earth; and, as often as the temperature of the solvent 
is lowered,mineral matter has a tendency to separate from it 
and solidify. Thus a stony cement is often supplied to sand, 
pebbles, or any fragmentary mixture. In some conglomer¬ 
ates, like the pudding-stone of Hertfordshire (a Lower Eo¬ 
cene deposit), pebbles of fimt and grains of sand are united 
by a siliceous cement so firmly, that if a block be fractured, 
the rent passes as readily through the pebbles as through 
the cement. 
It is probable that many strata became solid at the time 
when they emerged from the waters in which they were de¬ 
posited, and when they first formed a part of the dry land. 
A well-known fact seems to confirm this idea: by far the 
greater number of the stones used for building and road¬ 
making are much softer when first taken from the quarry 
than after they have been long exposed to the air; and 
these, when once dried, may afterwards be immersed for 
any length of time in water without becoming soft again. 
Hence it is found desirable to shape the stones which are to 
be used in architecture while they are yet soft and wet, and 
while they contain their “ quarry-water,” as it is called; 
also to break up stone intended for roads when soft, and 
then leave it to dry in the air for months that it may hard¬ 
en. Such induration may perhaps be accounted for by sup¬ 
posing the water, which penetrates the minutest pores of 
rocks, to deposit, on evaporation, carbonate of lime, iron, si- 
lex, and other minerals previously held in solution, and there¬ 
by to fill up the pores partially. These particles, on crystal¬ 
lizing, would not only be themselves deprived of freedom of 
motion, but would also bind together other portions of the 
rock which before were loosely aggregated. On the same 
principle wet sand and mud become as hard as stone wheti 
