CXX1V PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



granitic rocks are cited, the effects being similar to that produced on 

 granite in a glass-making furnace, the quartz alone resisting the 

 action of the heat ; but M. Delesse observes that, independently of 

 fusion more or less complete, felspathic rocks become impregnated 

 with " fer oligiste " and other sublimated products of volcanoes, and 

 have, like other rocks, been sometimes attacked by acid vapours. In 

 fact, metamorphosis, under such circumstances, depends on the 

 action and reaction between the active rock and the rock acted 

 upon, as also upon the effects produced by other agencies, due to the 

 eruption of gases and vapours, simultaneously with the principal 

 agent, the erupted rock. 



In calcareous rocks it is remarkable that, though in general they 

 are, by contact with lavas, rendered crystalline and granular, or in 

 some cases highly saccharoid, like marble, they appear on other 

 occasions to have undergone scarcely any alteration. This meta- 

 morphism, due to heat combined with pressure, has been well under- 

 stood since the experiments of Sir James Hall, confirmed as they 

 have been by those of several subsequent chemists : M. Delesse 

 observes, however, that neither a great amount of pressure or of 

 heat is necessary to produce it, and adduces as a proof the pro- 

 duction of crystalline stalactites, and even beds of a similar lime- 

 stone, without the aid of heat, — an example not, however, in my 

 opinion, analogous, as in this case the molecules have been enabled 

 to move by a previous solution of the body in water, whereas in the 

 other the cohesive attraction between the particles required the action 

 of heat to allow mobility. It appears to me, indeed, that the occa- 

 sional absence of the effects of metamorphic action indicates that in 

 such cases the pressure had not only prevented the escape of the 

 volatile constituents, but had also neutralized the effort of heat to 

 destroy the force of cohesion, and thus to allow the particles to 

 move amongst themselves, If so, it is due to some modification in 

 the circumstances, as yet to be explained by positive facts, though 

 easily guessed at as a speculation. Other more complex effects, such 

 as the introduction of magnesia into the rock, or (in the case of the 

 saccharoid limestone of Somma, specially studied and described by 

 M. Delesse) numerous minerals, are due to the reactions before 

 noticed in respect to felspathic rocks, the limestone being originally 

 marly or impure. Pure siliceous rocks, such as quartz, are acted 

 upon in a similar manner to granite, but have been less affected. 

 Fragments of quartz found imbedded in lava have become opake, 

 and have been fissured, but rarely melted, and then chiefly on the 

 surface. The presence of an alkaline solvent may, however, con- 

 tribute to a mere perfect solution ; and then the silica would be 

 absorbed in the mass of the lava. 



The effect of metamorphic action on argillaceous rocks is less 

 complete, as the surface may be vitrified, "and no alteration pro- 

 duced on the interior ; still, however, gaseous matter has in some 

 instances been expelled, and a cellular internal structure has been " 

 the result. M. Delesse then sums up the effects thus : either a cal- 

 cination more or less complete; the assumption of a stony, cellular, 



