ANNIVERSARY ADDRESS OF THE PRESIDENT. CXX111 



laceous, agglutinated together, tinted of various colours, ribanded 

 like jasper, still preserving traces of stratification, fragile, hard, and 

 sonorous, of which porcellanite is an example : and if the tempera- 

 ture he very high, they are melted and form vitrified substances, 

 just like the slags of a forge or furnace. These subterranean com- 

 bustions yield also volatile and sublimed products like volcanos, as, 

 for example, the combustible gases, vapour of water, specular iron, 

 sulphur, hydrochlorate of ammonia, and other salts. It is right, 

 however, to observe, that though the identity of the effects produced 

 by artificial calcination with those produced by the combustion of 

 beds of coal or lignite is fully established by these observations, it 

 can hardly be said that the conditions are those of rocks exposed to 

 intense heat under great pressure, and totally excluded from the air. 



This latter case is more nearly represented by the effects pro- 

 duced by lavas, which is the next subject of the inquiries of 

 M. Delesse, who names all those authors who had preceded him in 

 such investigations. At the Puy de Montehie, in Auvergne, trunks 

 of trees, more or less carbonized, are found imbedded in the volcanic 

 debris which forms the base of the Puy. The charcoal thus formed 

 is of a rich black colour, and of a metallic lustre ; it is friable, very 

 porous, light, and soils the fingers; it has preserved its original 

 woody structure, and by MM. Lecocq and Bouillet is considered to 

 have belonged to the birch. It is readily combustible ; and its com- 

 position, as determined by MM. Lecocq and Bouillet, is — carbon, 

 52-50 ; white ashes, consisting of carbonate of lime, with traces of 

 silica and alumina, 5*00 ; loss by ignition, 42*50 ; so that it is evi- 

 dent that the water and volatile matter had not been fully expelled 

 by the igneous rocks, to the action of which the combustible body 

 had been exposed. 



The composition of another example of wood carbonized by lava, 

 from Auvergne, M. Delesse found to be — carbon, 18*75 ; ashes, 46*48, 

 consisting of carbonate of lime, 6*21, oxide of iron with a little 

 alumina, 40*27; loss by ignition, 34*77. As charcoal formed arti- 

 ficially contains, when manufactured in damp air, not more than 20 

 per cent, of volatile matter, it is clear that, even in this example, cal- 

 cination under such circumstances has only partially expelled the 

 volatile matter of the wood, whilst both analyses show that the charcoal 

 has been impregnated with mineral substances ; so that the black or 

 red charcoal, produced by the action of the lava which, as at Vesuvius 

 where these effects may be studied, envelopes the wood or other vege- 

 table substances, is in a much more advanced stage of metamorphosis 

 than any artificially-formed charcoal. Felspathic bodies undergo very 

 material change from contact with lava : thus the granite of the 

 Roches Rouges, near the Puy, has assumed a prismatic structure ; 

 the fragments of granite of Denise, near the Puy (Haute Loire), im- 

 bedded in a volcanic scoria, have been partly melted, and have assumed 

 a cellular structure, the cavities being due to the dissipation of the 

 mica, which is the most fusible of the constituents of granite, whilst 

 the quartz, which is the least, remains distinguishable. 



Many other examples of the fusion, either partial or entire, of 



