146 LYELL'S ELEMENTS OF GEOLOGY. 



Rocks altered by Subterranean Gases. 



must suppose the influence of heat to be in some way connected 

 with the transmutation, if, for reasons before explained, we con- 

 cede the igneous origin of granite. 



The experiments of Gregory Watt, in fusing rocks in the 

 laboratory, and allowing them to consolidate by slow cooling, 

 prove distinctly that a rock need not be perfectly melted in order 

 that a re-arrangement of its component particles should take 

 place, and a partial crystallization ensue.* We may easily sup- 

 pose, therefore, that all traces of shells and other organic re- 

 mains may be destroyed ; and that new chemical combinations 

 may arise, without the mass being so fused as that the lines of 

 stratification should be wholly obliterated. 



We must not, however, imagine that heat alone, such as may 

 be applied to a stone in the open air, can constitute all that is 

 comprised in plutonic action. We know that volcanos in erup- 

 tion not only emit fluid lava, but give off steam and other heated 

 gases, which rush out in enormous volume, for days, weeks, or 

 years continuously, and are even disengaged from lava during 

 its consolidation. When the materials of granite, therefore, 

 came in contact with the fossiliferous stratum in the bowels of 

 the earth under great pressure, the contained gases might be un- 

 able to escape ; yet when brought into contact with rocks, might 

 pass through their pores with greater facility than water is 

 known to do. (See p. 52.) These aeriform fluids, such as sul- 

 phuretted hydrogen, muriatic acid, and carbonic acid, issue in 

 many places from rents in rocks, which they have discoloured 

 and corroded, softening some and hardening others. If the 

 rocks are charged with water, they would pass through more 

 readily ; for, according to the experiments of Henry, water, 

 under an hydrostatic pressure of ninety-six feet, will absorb 

 three times as much carbonic acid gas as it can under the ordi- 

 nary pressure of the atmosphere. Although this increased 

 power of absorption would be diminished, in consequence of the 

 higher temperature found to exist as we descend in the earth, 

 yet Professor Bischoff has shown that the heat by no means 

 augments in such a proportion as to counteract the effect of aug- 

 mented pressure."!" There are other gases, as well as the car- 

 bonic acid, which water absorbs, and more rapidly in proportion 

 to the amount of pressure. Now even the most compact rocks 

 may be regarded, before they have been exposed to the air and 

 dried, in the light of sponges filled with water ; and it is con- 

 ceivable that heated gases brought into contact with them, at 

 great depths, may be absorbed readily, and transfused through 



* Phil. Trans. 1804. t Poggendorf's Annalen, No. XVI., Second Series, vol. Hi. 



