ON THE EFFECTS OF LONG-CONTINUED HEAT. 189 



nace were appropriated to this object, it is not difficult to conceive a con- 

 struction and application of it which would fulfil such a requirement. 



If any one could succeed in effecting the synthesis of pseudomorphic 

 crystals, or of granites and porphyries, he would certainly perform a great 

 service to chemical geology. In the first of these subjects of experiment 

 success is scarcely to be looked for, except in the metamorphic action of 

 heated volatile agents. It is possible that granite also, and porphyry, might 

 be formed by a process of volatilization ; or they might perhaps be produced 

 as a residual igneous crystallization out of a mass, of which the flux had been 

 removed from the denser substances by sublimation, solution, or pressure. 



It should appear that the production of marble is also a problem still un- 

 determined. Rose has expressed an opinion, founded on his own ex- 

 periments, that the solid substance which Sir J. Hall obtained, by igniting 

 chalk under a pressure that prevented the extrication of the carbonic gas, 

 cannot have been marble. Possibly the presence of an excess of the acid 

 may be an additional requisite to the production of a perfect specimen. 



Since this Report was drawn up, I have seen a memoir by M. Daubree* 

 which contains a very able and complete exposition of the progress of 

 geological chemistry. His observations on the deposit of zeolitic crystals 

 and other minerals discovered in the interstices of the old Roman brick- 

 work and concrete at Plombieresf, which have undergone the action of sili- 

 cated waters springing from the earth at a temperature not, now at least, 

 exceeding 158° F., seem to have solved the problem of the deposit of such 

 crystals and minerals in the vesicles of basaltic rocks, and to have proved 

 them to be due to aqueous infiltration whilst the rock was still hot. 



His views on the formation of another class of minerals, and the origin of 

 the granitic and other early rocks, seem to be not equally satisfactory. To 

 these he has been led by his own late experiments on the effect of aqueous 

 vapour in decomposing obsidian and glass. He propounds, with the diffidence, 

 however, which belongs to a hypothetical speculation, a theory to the 

 following effect — that in a primaeval state of the earth, when the heat now 

 known to exist in its interior extended to the surface, as that surface cooled 

 down to a certain point, the red-hot obsidian, or silicated glass, of its first 

 coat was decomposed by water condensed from a state of vapour, under 

 great pressure, at a red heat ; thus the quartziferous rocks were formed, at 

 first as a plastic sponge, and when the water had evaporated as granite, the 

 schist and slates immediately superincumbent upon it being the residuary 

 product of the mother-waters. 



But this speculation is open to grave objections. What principle of 

 solidification, it may be asked, capable of compacting granite, is included 

 in a process of disintegration ? What has become of the silicates involved 

 in it, to which we might look for such solidification, but which are absent 

 from granite ? The mother -water* which it supposes are incapable of dif- 

 fusing the peculiar minerals encysted in the proximity of granitic rocks 

 even to the distance of thousands of feet. No less unaccountable would be 

 the absence of all the zeolitic and opaline substances that might have been 

 expected. Everything tends to show that whatever the power of this process 

 may be, it must be confined, at least, to the lavas, basalts, and trachytes. 



That heated water has been so universal a solvent as M. Daubree supposes, 

 is rendered very improbable by a circumstance noticed by Cagniard de 

 Latour in his celebrated experiments on vapour highly heated and com- 



* Etudes et experiences synthetiques sur le metamorphisme et surla formation des roches 

 crystallines, 18G0. 



t The presence of fluorine in the apophyllite of Plombieres is remarkable, the more be- 

 cause Vauquelin analysed the waters with the express object of detecting this constituent, 

 and denied its supposed existence in them. 



