276 EXPERIMENTS ON METAMORPHISM AND 



rectly connected with the subject which occupies us; as the precipita- 

 tion of iron ores in bogs, the production of nitrates, the formation of 

 carbonate of soda at the bottom of hikes and that of chiy stones, the de- 

 composition of pebbles into kaolin, &c. But it would be difficult here 

 to embrace in their whole extent these incipient transformations which 

 are, as it were, the life of the inorganic world. I shall confine my- 

 self to activities of a deeper origin. 



THIRD PART. 



THEORETICAL CONSIDERATIONS ON THE CAUSE OF META- 

 MORPHIC PHENOMENA: SYNTHETICAL EXPERIMENTS IN 

 SUPPORT OF THEM. 



In the theoretical considerations which I am about to present, I 

 shall first treat of those rocks whose metamorphic origin is demon- 

 strated by the circumstances in which they are found. The oldest 

 crystalline rocks, which have also been sometimes considered as 

 metamorphic, but without the same corroborative proofs, will be the 

 subject of an appendix which terminates this essay. With regard to 

 dolomites, and the rocks Avhich are connected with them, I shall add 

 nothing to the facts and explanations which I have noticed in the 

 second part. 



CHAPTER I. 



INTEENAL HEAT ; ITS EFFECT ON METAMORPHISM ; IT IS NOT SUFFICIENT TO 

 EXPLAIN ALL THE PHENOMENA. 



The modifications undergone by the formations comprised under 

 the name of metamorphic have incontestably taken place at a higher 

 temperature than that which we now have on the surface of the 

 globe. We can infer it, in the first place, from the single fact of the 

 mineralogical analogies which these formations have with the erup- 

 tive rocks, and especially the presence of numerous anhydrous sili- 

 cates, which forms one of their most remarkable features; in the 

 second place, from their evident relation to the dislocations, whose 

 starting point is always in the interior regions of the earth, and 

 which incontestably are referred tb the internal heat of the globe as 

 a first cause. 



The heat of the globe necessarily decreases from the centre 

 towards the surface, and consequently when the sediments deposited 

 in the ocean at the relatively low temperature which exists in its 

 deeper parts were afterwards covered by other beds, they must 

 have acquired a greater degree of heat by reason of their greater 



