MINERALIZATION OF ORGANIC REMAINS. 
65 
filled with water under great pressure as they sink, other¬ 
wise they would be immediately crushed to pieces and flat¬ 
tened. Nevertheless, if the materials' of a stratum remain 
in a yielding state, and do not set or solidify, they wdll be 
gradually squeezed down by the weight of other materials 
successively heaped upon them, just as soft clay or loose 
sand on which a house is built may give way. By such 
downward pressure particles of clay, sand, and marl may be¬ 
come packed into a smaller space, and be made to cohere to¬ 
gether permanently. 
Analogous effects of condensation may arise when the 
solid parts of the earth’s crust are forced in various direc¬ 
tions by those mechanical movements hereafter to be de¬ 
scribed, by which strata have been bent, broken, and raised 
above the level of the sea. Rocks of more yielding materi¬ 
als must often have been forced against others previously 
consolidated, and may thus by compression have acquired a 
new structure. A recent discovery may help us to compre¬ 
hend how fine sediment derived from the detritus of rocks 
may be solidified by mere pressure. The graphite or black 
lead ” of commerce having become very scarce, Mr. Brocke- 
don contrived a method by which the dust of the purer por¬ 
tions of the mineral found in Borrowdale might be recom¬ 
posed into a mass as dense and compact as native graphite. 
The powder of graphite is first carefully prepared and freed 
from air, and placed under a powerful press on a strong steel 
die, with air-tight fittings. It is then struck several blows, 
each of a power of 1000 tons; after which operation the 
powder is so perfectly solidified that it can be cut for pen¬ 
cils, and exhibits when broken the same texture as native 
graphite. 
But the action of heat at various depths in the earth is 
probably the most powerful of all causes in hardening sedi¬ 
mentary strata. To this subject I shall refer again when 
treating of the metamorphic rocks, and of the slaty and joint¬ 
ed structure. 
Mineralization of Organic Remains. —The changes which 
fossil organic bodies have undergone since they were first 
imbedded in rocks, throw much light on the consolidation 
of strata. Fossil shells in some modern deposits have been 
scarcely altered in the course of centuries, having simply 
lost a part of their animal matter. But in other cases the 
shell has disappeared, and left an impression only of its ex¬ 
terior, or, secondly, a cast of its interior form, or, thirdly, a 
cast of the shell itself, the original matter of which has been 
removed. These different forms of fossilization may easily 
