ON THE FORMATION OF CRYSTALLINE ROCKS. 261 



When we can succeed in thus taking nature by surprise, after the 

 first emotion of pleasure at having wrested from her one of her 

 secrets, we experience a feeling of humility in seeing at the cost of 

 what difficulties we are able to reproduce a few of the most simple 

 mineralogical formations; yet the results already attained show that 

 we have no reason to be discouraged, and that we have it in our power 

 to imitate no small number of minerals without the intervention of 

 centuries. 



SECOND PART. 



EXPOSITION OF THE FACTS WHICH CONJOINTLY CON- 

 STITUTE METAMORPHISM. 



The variety of the facts which I have to present induces me to 

 group them in five chapters. I shall first examine metamorphism of 

 JuxtajMsitiom; secondl}', regional meiamorphism. I shall then pass in 

 review metamorphism of structure] i\\en the phenomena relating to the 

 deposits of dolomite, rock salt, sulphur, and bituminous deposits; finally, 

 the general relation of metaUiferous deposits and thermal springs to met- 

 amorphism. 



The observations relating to primitive gneiss, the origin of which is 

 doubtful, will be reserved for the third part, which will terminate 

 this memoir. 



This exposition will be very brief; for it has for its object well- 

 known facts, which it is only necessary to recall in order that Ave bear 

 in mind the conditions which the theoretical explanation ought to 

 satisfy.* 



FIRST CHAPTER. 

 METAMORPHISM OF JUXTAPOSITION, f 



When a rock has been thrown up from below, the beds which it 

 traverses have generally been modified in its neighborhood. Some- 

 times this modification of the enclosing rocks is reduced to a very 

 narrow bordering, of a few sixteenths of an inch, and the changes 

 produced upon this thin layer are not very decided, J In others, 



^ The examples luiowa would form an entire treatise on descriptive geology. I shall 

 not give them in detail, but content myself with simply announcing them. I shall refer 

 for details to the best works on general geology, particularly to those of Naumann, Studer 

 and Lyell. 



-j- It seems best to me to use this denomination rather than that of metamorphism of 

 cont'id, which is ordinarily employed, beciuse the modifications which it explains extend 

 far beyond the contact of the rocks. The word local does not appear to me to be suffi- 

 ciently characteristic. 



I As an example, I will confine myself to citing manj^ veins of basalt which intersect 

 the Jurassic formation of the Wurtemberg Alps. Granite itself has not always modified 

 the schist, even when it was in a sufficiently fluid state to be injected into it in veins, as 

 in the Vosges near Wesserling. — {Bulletin de la Sociele Geologiqve, vol. iv, p. 144(i.) 



