288 EXPERIMENTS ON METAMORPHISM AND 



if we consider the great extent and the remarkable uniformity of its 

 action ? 



Before examining this latter agency of water, it seems natural to 

 examine to what extent its presence is possible in rocks. Let us first 

 say that it follows, from the experiments already cited, that a very 

 small quantity of water only is necessary to produce, under favorable 

 conditions of pressure and temperature, ver}^ marked changes. We 

 cannot, in effect, see without astonishment that such a complete trans- 

 formation in the chemical and physical state of glass can be obtained 

 by a quantity of water equal to about a third of its weight. This 

 shows us how the water of constitution of some rocks, as the clays 

 for instance, was sufficient to produce metamorphism when heat gave 

 it the power to react upon the elements with which it was associated. 



With regard to those rocks which do not contain any Avater of con- 

 stitution, let us first make the remark that many of them are destitute 

 of the water called quarry -ivater. We cannot suppose that this water 

 has been lodged otherwise than in the pores of the rock. All rocks 

 are, therefore, porous; and what takes place in the artificial colora- 

 tion of agates proves that stones which are in appearance perfectly 

 compact are by virtue of the sole force of capillarity penetrated by 

 a liquid. 



We cannot deny that if water can insinuate itself, by crevasses, 

 into the solid crust of the globe at a depth only equal to that of the 

 sea, it acquires a pressure of several hundred atmospheres, by the 

 aid of which it penetrates more easily, perhaps, into the most 

 minute pores of the rocks, especially at the temperature which it 

 possesses at such a depth. This action is, without doubt, aided by 

 capillarity within limits of which we can form no idea. If, however, 

 the rocks Avere altogether impermeable, provided the water be 

 endowed with the power of attacking their surface, time only is 

 necessary in order that its action should be gradually propagated to 

 considerable distances. Indeed, in the tubes prematurely withdrawn 

 I was able to see that the attack had taken place by successive layers, 

 in such a way that there yet existed between the two surfaces of the 

 glass a portion that was transparent and altogether unaltered. Thus, 

 whether the water of rocks is that of constitution or penetration, we 

 are justified in expecting, as soon as the temperature is sufficiently 

 raised, reactions comparable to those produced in our experiments, 

 as well as in eruptive rocks. This, however, will be verified by an 

 example as conclusive as surprising, which abounds in new facts, and 

 has the double merit of being contemporaneous, and of having been 

 performed under conditions which are now perfectly well known. 



CHAPTER VI. 

 CONTEMPORANEOUS METAMORPHISM OF PLOMBIERES. 



The concrete which the Romans put down nearly to the points of 

 emergence of the thermal springs is composed of fragments of brick 



