ANNIVERSARY ADDRESS OF THE PRESIDENT. CXX1 



may stand revealed. At this point it may be observed that M. 

 Delesse soon convinced himself that the effect of heat as an agent 

 of metamorphism has been mnch exaggerated, and even that porcel- 

 lanite, so often assimilated as an igneous product to the vitreous 

 slags of furnaces, which are produced by the action of beds of coal 

 in a state of ignition, has only very partially owed its production 

 to heat. Nor is this all, as he considers that the same remark 

 applies to various other rocks, both metamorphic and eruptive, the 

 formation of which has been exclusively ascribed to heat. 



To solve a question of this kind, M. Delesse first submitted the 

 various rocks to the action of heat in the laboratory, as the effects 

 upon some substances may be very great, and upon others very slight. 

 The prismatic structure which the lining of furnaces assumes by 

 heat has often been noticed, as also the vitrified scoriaceous con- 

 dition assumed by many substances ; but all these bear about them 

 marks of their origin which are easily recognized. As the action 

 of heat necessarily varies with the composition of rocks, M. Delesse 

 briefly notices the peculiarities of metamorphism which they ex- 

 hibit : thus, combustibles lose their water and volatile constituents, 

 and are converted into charcoal or coke, but are never by heat 

 alone transformed into anthracite, — the temperature necessary for 

 producing these effects being much less than is usually supposed, 

 as the heat required for distillation of the bituminous matters ranges 

 from 500° in turf and 572° in lignite, to 752° in coal and anthra- 

 cite, or in all cases a temperature below red heat. On this point 

 M. Delesse observes that, admitting the progressive increase of 

 temperature in penetrating towards the interior to be at the uniform 

 rate of 1° for 60 feet, the highest temperature above named would be 

 possessed by a rock erupted from a depth of about 9-1- miles, — whilst 

 the great change produced by the expulsion of the larger portion of 

 the water of the combustible substances being effected at a tempera- 

 ture little above 212°, the eruption of a rock from a depth of 2\ 

 miles would be sufficient to produce such an effect. I need scarcely 

 observe that this reasoning simply implies that an increase of tem- 

 perature proportionate to the depth below the surface of the earth 

 is an observed fact, and leaves untouched the question as to the true 

 cause of that increase. It is only necessary to name the other agen- 

 cies which M. Delesse cites as capable of abstracting the bituminous 

 and volatile matter of combustibles, such as benzine, itself a product 

 of the distillation of coal, alkalies and alkaline carbonates, and highly 

 heated currents of water, charged with mineral and especially alka- 

 line matter, traversing the beds of combustible matter when situated 

 deep in the interior. M. Delesse indeed concludes from this latter 

 fact, that the difference between lignite, coal, anthracite, and graphite 

 is rather due to the metamorphic action produced by this aqueous 

 action than to a simple dry distillation. 



Pure calcareous rocks would lose by heat alone their carbonic acid, 

 and be brought as caustic lime into a condition to undergo further 

 change ; but marls or impure limestones might enter into fusion, 

 and thereby undergo a complete change. Sulphate of lime, even at 



