248 COSMOS. 



of the &oa. This extended insight into the coi 3:;tion of nat. 

 ural phenomena is the result of the philosop lical direction 

 which has been so generally assumed by the more earnest 

 study of geognosy. Increased cultivation of science and en- 

 largement of political views alike tend to unite elements that 

 had long been divided. 



If, instead of classifying rocks according to their varieties of 

 form and superposition into stratified and unstratified, schistose 

 and compact, normal and abnormal, ^ve investigate those phe- 

 nomena of formation and transformation which are still going 

 on before our eyes, we shall find that rocks admit of being ar- 

 ranged according to four modes of origin. 



Rocks of eruption, which have issued from the interior of 

 the earth either in a state of fusion from volcanic action, oi 

 in a more or less soft, viscous condition, from Plutonic action. 



Sedimentary rocks, which have been precipitated and de- 

 posited on the earth's surface from a fluid, in which the most 

 minute particles were either dissolved or held in suspension 

 constituting the greater part of the secondary (or fiotz) and 

 tertiary groups. 



Transformed or metajnorphic 7'ocks,^ in M'hich the internal 

 texture and the mode of stratification have been changed, ei- 



* [As the doctrine of mineral metamorphism is now exciting very 

 general attention, we subjoin a few explanatory observations by the 

 celebrated Swiss philosopher, Professor Studer, taken from the Edinh. 

 New Philos. Journ., Jan., 1848: " In its widest sense, mineral meta* 

 morphism means every change of aggregation, structure, or chemical 

 condition which rocks have undergone subsequently to their deposition 

 and stratification, or the effects which have been produced by other 

 forces than gi-avity and cohesion. There fall under this definition, the 

 discoloration of the surface of black limestone by the loss of carbon ; 

 the formation of brownish-red crusts on rocks of limestone, sandstone, 

 many slate stones, serpentine, granite, &c., by the decomposition of iron 

 pyrites, or magnetic iron, finely disseminated in the mass of the rock ; 

 the conversion of anhydrite into gypsum, in consequence of the absorp. 

 tion of water ; the crumbling of many granites and porphyries into 

 gravel, occasioned by the decomposition of the mica and feldspar. In 

 'ts more limited sense, the term metamorphic is confined to those 

 changes of the rock which are produced, not by the effect of the at- 

 mosphere or of water on the exposed surfaces, but which are produced, 

 directly or indirectly, by agencies seated in the interior of the earth. 

 In many cases the mode of change may be explained by our physical 

 or chemical theories, and may be viewed as the effect of temperature 

 or of electro-chemical actions. Adjoining rocks, or connecting com^ 

 munications with the interior of the earth, also distinctly point out the 

 Beat from which the change proceeds. In many other cases the meta 

 raoi'phic process itself remains a mystery, and from the nature of the 

 products alone do v.'e conclude that such a metamorphic action has 

 taken place.] — Tr, 



