DELESSE- — ORIGIN OE ROCKS. 6 



■ which contribute to the formation of rocks ; and first we come to the 

 consideration of water, which claims very special attention. 



§ 4. Water. — "When we penetrate into the interior of the earth 

 we usually meet with water. This subterranean water forms 

 sheets which often occur in stages one below the other, and which 

 continue to very great depths. It represents assuredly a very 

 notable portion of the water which exists on our planet. It is 

 otherwise concealed ; and it is without doubt for this reason that 

 it has not attracted attention, and that geologists have not accorded 

 to it all the importance which it deserves. It is evident, however, as 

 Bischoff has remarked, that it must interfere in all the phenomena 

 which take place in the interior of the earth. 



Let us seek, then, to appreciate the effects of this subterranean 

 water. In the first place, in proportion as we penetrate into the 

 earth, its temperature becomes successively elevated ; the materials 

 which it holds in solution augment little by little with the depth ; and 

 when its temperature is very high, it ought to exercise very energetic 

 chemical reactions on the rocks with which it is in contact. At all 

 depths the materials which it penetrates would easily obey the mole- 

 cular actions which incite them. The most favourable circumstances 

 here, then, are found combined for the development of minerals. 



The following topics are then discussed : — 



§ 5. A rock contains water when it is in the interior of the earth. 



§6.-4 rock, fusible or infusible, can become plastic by the agency 

 of water. 



On this latter subject the author observes : When a fusible or 

 infusible rock is impregnated with water, it always experiences a 

 certain softening, and sometimes even becomes entirely plastic. Re- 

 ciprocally, when it loses its water it becomes on the contrary more or 

 less lithoid. 



This action exercised by water on the physical properties of rocks 

 is extremely important, and it ought not for a single instant to be 

 lost sight of in researches like those with which we are now occu- 

 . pied ; for in the interior of the earth all rocks are humid, and con- 

 sequently very different from what they arc at the surface. The con- 

 dition in which we know them in our collections is also in some sort 

 exceptional. At a little depth, the water is moreover aided in its 

 effects by heat and pressure ; consequently these three causes combined 

 ought to assist each other in reducing the rocks to a plastic state. 



§ 7. Pressure. — Pressure, as Nauinann rightly observes, has neces- 

 sarily exerted an influcnec in the formation of minerals and of the 

 rocks which compose the terrestrial crust. One comprehends indeed 

 that in the interior of the earth the rocks are subjected to a very 

 considerable pressure, owing to the thickness of the strata which 

 cover them. 



If one considers now the eruptive rocks, they must have sustained 

 considerable pressures resulting from the force itself which elevated 

 them to the surface. They have also been compressed by the lateral 

 pressures produced upon their walls by the enclosing rock. 



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