CXX PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



necessarily induce a very great change in its ultimate condition, quite 

 independent of the possible introduction into its mass of other ele- 

 mentary constituents derived from the enveloping strata. Here, 

 again, the examination of the reciprocal effects produced by the pas- 

 sage of erupted rocks through various substances has the advantage 

 of not only showing the difference of the results produced on the 

 enveloping rocks, but also on the erupted rocks themselves, which, 

 however different they appear in their final condition, may possibly 

 be reduced to the same original by such inquiries. As M. Delesse 

 observes, quoting an old proverb, " just as the workman is known 

 from his work," so also from the study of metamorphosed rocks may 

 be traced out the true history of the erupted rocks which have pro- 

 duced them. 



M. Delesse begins his inquiry with direct metamorphism, or that 

 change produced on the enveloping strata by the erupted rock, and 

 limits himself first to the effects produced by the eruption of truly 

 siliceous rocks, such as lavas, trap-rocks, and granites. The term 

 lava is restricted to those flowing mineral masses anhydrous in com- 

 position and manifestly of igneous origin. Trap -rocks are hydrated 

 mineral masses, the base of which is a hydrated felspar of the sixth 

 system, anorthose,and which may be therefore called anorthose rocks : 

 they include basalt, dolerite, hyperite, euphotide, trap, diorite, 

 hornblende-rock, greenstone, kersantite, &c, to which are added 

 " Sherzolithe" and serpentine. Granite-rocks contain, as an essen- 

 tial element, orthose felspar, and may be therefore called orthose- 

 rocks : they include granite (as the type), syenite, protogine, por- 

 phyry, eurite, minette, and even gneiss. The family of trachytes, 

 containing vitreous felspar, belongs to the orthose-rocks ; but, as some 

 of them are hydrated rocks, such as retinite, perlite, phonolite, and 

 even obsidian, the metamorphism they have produced differs in 

 character from that of granitic rocks, and they must therefore be 

 classed with the trap-rocks, so that all hydrated volcanic rocks are 

 grouped together as traps. The metamorphism produced by granitic 

 rocks is not so easily traced out as that proceeding from the action of 

 trap rocks, since it generally happens that the enveloping rocks are 

 also crystalline, and that it becomes therefore no easy matter to 

 determine the line of demarcation between the granitic nucleus and 

 the granitic envelope, or to distinguish between the action of special 

 and of general or normal metamorphism. M. Delesse therefore 

 begins by explaining the metamorphic action of trap -rocks ; and he 

 investigates the effect produced on mineral substances such as the 

 ores of iron, on combustible substances, on felspathic rocks, on lime- 

 stones, and on siliceous and on argillaceous rocks, first citing the actual 

 facts he has either observed himself in nature or in well- authenticated 

 specimens, then describing the erupted rock and bringing into com- 

 parison the original and the metamorphosed condition of the rock 

 upon which it has acted, — the principal mineralogical and chemical 

 characters of all these being carefully determined, so that both the 

 physical changes of structure and density, as well as the alteration 

 of composition from the loss of particular elementary substances, 



