FOREIGN CORRESrONDENCK. 



245 



a couple of yards from the sides of tlie erupted trap-rock. In c-eneral, 

 this change has been gTeatest where basalt and dolerite haxc hcen 

 ejected. The nietamorphism undergone by limestone, in these circimi- 

 stances, includes a change of structure and the formation of certain 

 minerals. Some calcareous strata have become hard and compact in 

 those parts which have been j)ushed up b}^ trap-rocks ; others have 

 taken a fi-agmentary, or even a prismatic structiu^e — but only when 

 they are argillaceous or siliceous. The prisms thus formed in. limestones 

 are not so well defined as those we observe in other varieties of rocks ; 

 for mstance, in sandstones, Szc. The limestone of the Pyrenees has 

 become cavernous or cellidar by the metamorphic action of diorite. 

 "Limestone," says M. Delesse, "generally becomes crj^stalline by 

 nietamorphism ; its colour becomes paler, and often exceedingly 

 white." This reminds us that, some time ago, we found that certain 

 limestones, Avliich Avere remarkable for their whiteness, and which had 

 been modified in structure by metamorphism, contained a notable 

 quantity of magnesia, which must perhaps be attiibuted to the contact 

 of the eruptive rock. The analysis, made at the same tune, of a 

 gTeenish-yellow compact dolomite gave me protoxide of iron and 

 magnesia, besides lime, in such proportion that a certain quantity of 

 protoxide of iron seems to have been substituted for an equivalent 

 quantity of magnesia. 



When a trap-rock has exercised metamoi'phic action iipon a stratum 

 of limestone, M, Delesse observes that the densitj^ of the latter has 

 augmented considerably ; and in the vicinity of the eruptive rock are 

 found mineral species produced by its influence, amongst others, 

 oxide of iron, oxide of manganese^ magnetic j^yrites, brucite, dolomite, 

 calcareous spar, chlorite, p^a^oxene, garnet, idocrase, gehlenite, &c. We 

 find, also, in veins, mjected, as it were, through the limestone, 

 barytine, celestine, oligiste, pyrites, galena, blende, copper-pyrites, and 

 some others. 



M. Delesse has observed, also, that gypsum has been acted upon 

 by metamorphism much in the same way as limestone ; thus, it 

 has been observed in some cases to have become crystalline, and to 

 contam oligiste and even iron-glance (carbonate of iron). We Avill 

 reserve for a future paper M. Delesse's remarks on the metamorphism 

 of sandstones and argillaceous rocks. 



Professor Scacclii, of Xaples, has lately addressed a letter to the 



