364 



On Mineral Metamorphism. 



As the doctrine of mineral metamorphism is now exciting 

 very general attention, we, as introductory to what we may 

 afterwards communicate on this important and curious sub- 

 ject, lay before our readers a few explanatory observations by 

 one of the most distinguished of modern geologists — the 

 celebrated Swiss philosopher and professor, B. Studer.* 



Metamorphism, Definition. 



In its wider sense, mineral metamoi'phism 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 gravity 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 slatestones, 

 serpentine, granite, and so on, by the decomposition of iron 

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

 the rock ; the conversion of anhydi'ite into gypsum, in con- 

 seauence of the absorption of water ; the crumbling of many 

 granites and porphyries into gravel, occasioned by the decom- 

 position of the mica or felspar. In this doctrine must also 

 be reckoned the conversion of water into ice, and of snow and 

 ice into water, or into steam. In its more limited sense the 

 term metamorphic is confined to those changes of the rock 

 which are produced, not by the effect of the atmosphere or of 

 water on the exposed surfaces, but which are produced, di- 

 rectly 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. Ad- 

 joining rocks, or connecting communications with the interior 

 of the earth, also distinctly point out the seat from which the 



* Vide Lehrbuchder Physikalchen Geographie und Geologie, vol. 2. Bern, 

 1847. 



