FOREIGN CORRESPONDENCE. 



109 



la one of his memoirs the author treats exclusively of the changes 

 undergone by combustibles, such as lignite, coal^ Sec, 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 metamorpJiism. 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 hi/ 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. 



Eut 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. Tor instance, the 

 coal-measures of La Pleau, in Trance, 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. Tip to 

 the present time, according to M. Delesse, no cole 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 



