Geology, as illustrated by Chemistry and Physics. 53 



ing to Jeffrey's experiments, aqueous vapour decomposes 

 feldspathic rocks at the melting point of cast iron, and coats 

 the roof of the furnace with a porous covering of silica. Aque- 

 ous vapours, therefore, act only as decomposing agents, and 

 in no case produce crystalline formations. 



Gaseous exhalations from the interior of the earth, — carbo- 

 nic acid the most frequent, sulphuretted hydrogen the less 

 frequent, sulphurous acid and hydrochloric acid, which are 

 rare and occur only during volcanic eruptions, — exercise also 

 a decomposing influence. 



Although no trace of feldspar crystals has yet been found 

 in the slate fragments imbedded in scoriaceous masses, still 

 they occur in sedimentary formations ; for instance, at many 

 places in the Swiss Alps, far distant from all crystalline 

 feldspathic rocks. Thus, according to Studer,* white quartz 

 rocks, which pass into gneiss, lie in Churwalden in Bundten 

 upon grey slate beds, several thousand feet in thickness, whose 

 organic remains distinctly prove their sedimentary forma- 

 tion. Upon this quartz follows, as the uppermost covering, 

 grey slate again. Here, therefore, the metamorphic gneiss 

 is separated from crystalline rocks, as well as from the high 

 temperature of the earth's interior, by thick strata of un- 

 altered rocks. Neither those crystalline rocks, if supposed 

 to have ascended eruptively, i. e., in a melted state, nor the 

 central heat of the earth, could, in this and many similar 

 cases, have effected the metamorphosis, even if such a change 

 were possible. 



The wandering fancy of those geologists who maintained, 

 and still maintain that the metamorphosis of sedimentary 

 rocks was effected by their contact with crystalline rocks, 

 went so far that they affirmed such an action to take place 

 even at distances of 6000 feet, as if heated rocks acted like 

 miasmas. t These same geologists, in many other cases, freely 

 admit that the action of eruptive rocks upon the contiguous 

 strata, either cannot be at all recognised, or only in a very 

 slight degree. Studer, who, among other great merits, has 



* Lehrbuch der Physikal. Geographie und Geologic. Vol. ii., p. 137. 

 t Lehrbuch der Chein. u. Physikal. Geologic Bd. II., p. 353. 



