FOREIGN CORRESPONDENCE. 
109 
In one of his memoirs the author treats exclusively of the changes 
undergone by combustibles, such as lignite, coal, &c., under the modi- 
fying influence of some eruptive rock. In nature we find that fossil-wood 
modified by metamorphic action has become coal ; coal, anthracite ; and 
anthracite, graphite. Sometimes such transformations have taken place 
on a very large scale ; an entire stratum of coal, for instance, has been 
transformed, by the action of some upheaved rock, into hard anthracite. 
M. Delesse calls this normal metnmorphism. At other times, on the 
contrary, metamorphism is only observed to have occurred in that par- 
ticular part of the combustible stratum which is immediately in contact 
with the eruptive rock. This is metamorphism lij contact. 
In normal metamorphism the combustible loses successively its bitu- 
minous matter, becomes comparatively richer in carbon, more compact, 
and augments in density. Last of all it become crystalline, passing to 
the state of graphite. 
But in metamorphism by contact the changes are more complicated, 
and depend, in great measure, on the particular kind of eruptive rock 
that produces them. When lava, vomited by a volcano, flows over and 
envelopes pieces of wood, the wood becomes, in a greater or lesser 
degree, perfect coal. Some carbonized wood found in the ancient lava 
of Auvergne contained a certain quantity of carbonate of lime and oxide 
of iron, showing, perhaps, that in such cases the combustible matter 
may be impregnated with mineral substances. It is not often that we 
see granite and quartzose porphyry in contact with combustibles ; but 
some few cases of such contact have been observed. For instance, the 
coal-measures of La Pleau, in France, spoken of by Beudant, which 
are in some parts imbedded in the porphyroid granite of this district, 
and the coal-strata of Altwasser, in Silesia, visited lately by M. Delesse, 
where the coal in contact with porphyry has been changed into pris- 
matic anthracite, and furnishes, by combustion, 15 per cent, of ashes, 
principally formed of oxide of iron. In cases where combustibles are 
found encased in granite-rocks, they are observed to have lost their 
bituminous matter, and to have become anthracite or graphite. Up to 
the present time, according to M. Delesse, no cohe has ever been found 
in contact with granite-rocks ; it seems probable, however, that laminae 
of graphite found in them have been produced by their metamorphic 
action on certain combustible or bituminous matters. 
Trap-rocks, as basalt, dolerite, hyperite, euphotide, diorite, and 
