No. 161,] 
203 
theory, or rather hypothesis correct, of igneous injection from be- 
low. 
That changes innumerable after the consolidation of rocks have 
gone on, are well known to most geologists. No fact is more 
common, than the substitution of crystallized carbonate of lime, 
for the organic arrangement of testaceous and other lapideous bo 
dies. Mineral substitutions of quartz, pyrites and many other sub- 
stances, have numbe^*less examples, both on a small and on a large 
scale. In the mining region of Temoxcaltepec, in Mexico, where 
a great number of the mines are in argillite containing carbonate 
of lime, the stony part of the veins or matrix is composed of quartz, 
and carbonate of lime. If the rock be decomposed, and the instan- 
ces are numerous, and often for hundreds of feet in depth, the 
whole of the carbonate of lime of the vein is replaced by quartz, 
the latter often presenting by substitution, the forms belonging to 
the calcareous mineral. In those decomposed veins, the metallic 
minerals exist in combinations wholly different from those which 
they exhibited in the undecomposed state of the veins. 
The tendency of similar particles to unite to each other, is in 
proportion to the force of crystallization, exemplified by the fre- 
quency of form, and structure of the masses, of any given materi- 
al. So powerful is this force, that though particles of carbonate 
of lime in crystallizing cannot force sand from their sphere, as is 
evidenced in the Fontainbleu gres, yet sand is no obstacle to the 
assumption of their form. Mud offers no resistance to crystalliza- 
tion, as experiments in our laboratories, of alum and other salts, 
fully prove. This fact is often exhibited in nature, and by many 
mineral substances; but in none more beautifully exemplified, than 
by common or rock salt: the salt having separated from the marl 
which surrounds or envelopes it, whilst the latter was in a dessi- 
cating state. So also in the highly interesting deposition of gyp- 
sum at the outlet of Seneca lake; there this mineral is seen, not 
in beds, like all mechanical or sedimentary depositions, but in de- 
tached masses, with rounded outline, separated from each other, 
and enclosed by the indurated marl, each mass of gypsum having 
acted as a centre of crystallization, ejecting the marl and collect- 
ing to itself all those kindred particles within the range of its 
sphere. Similar illustrations are likewise exhibited in the ^emi- 
crystallized matter of the nodules of the flint in chalk; in the horn- 
stone of the cornitiferous limerock of Professor Eaton — in the con- 
