256 COSMOS. 



careous sandstone (Macig7io), which contains algsB found in 

 the northern Apennines, often assume a new and more brill- 

 iant appearance after their metamorphosis, which renders il 

 difficult to recognize them. The theory of metamorphism 

 was not established until the individual phases of the change 

 were followed step by step, and direct chemical experimenta 

 on the difference in the fusion point, in the pressure and time 

 of cooling, were brought in aid of mere inductive conclusions. 

 Where the study of chemical combinations is regulated by 

 leading ideas,* it may be the means of throwing a clear light 

 on the wide field of geognosy, and over the vast laboratory of 

 nature in which rocks are continually being formed and mod- 

 ified by the agency of subterranean forces. The philosophical 

 inquirer will escape the deception of apparent analogies, and 

 the danger of being led astray by a narrow view of natural 

 phenomena, if he constantly bear in view the complicated 

 conditions which may, by the intensity of their force, have 

 modified the counteracting effect of those individual substan 

 ces whose nature is better known to us. Simple bodies have, 

 no doubt, at all periods, obeyed the same laws of attraction, 

 and, wherever apparent contradictions present themselves, T 

 am confident that chemistry will in most cases be able to 

 trace the cause to some corresponding error in the experiment. 

 Observations made with extreme accuracy over large tracts 

 of land, show that erupted rocks have not been produced in an 

 irregular and unsystematic manner. In parts of the globe most 

 remote from one another, we often find that granite, basalt, and 

 diorite have exercised a regular arid uniform metamorphic ac- 

 tion, even in the minutest details, on the strata of arjjillaceous 

 slate, dense limestone, and the grains of quartz in sandstones. 

 As the same endogenous rock manifests almost every where the 

 same degree of activity, so, on the contrary, different rocks be- 

 longing to the same class, whether to the endogoiious or the 

 erupted, exhibit great differences in their character. Intense 

 heat has undoubtedly influenced all these phenomena, but the 

 degree of fluidity (the more or less perfect mobility of the parti- 

 cles — their more viscous composition) has varied very consid- 

 erably from the granite to the basalt, while at different geo- 



* See the admirable researches of Mitscherlich, in the Ahhandl. dei 

 Berl. Akad. for the years 1822 and 1823, s. 25-41 ; and in Poggend., 

 Annalm, bd. x., s. 137-152; bd. xi., a. 323-332; bd. xli., s. 21^3-21^ 

 (Gustav Rose, Ueher Bildung des Kalkspaths U7id Aragonits, iu Pog- 

 gend , Annalen, bd. xli., s, 353-366 ; Huidinger, in the Transactions 

 tfthe Royal Society of Edinburgh, 1827, p. 148.) 



