52 Observations on the Petrifaction of Shells 



have concurred in the petrifaction of the organized bodies of the old 

 world. The peroxide of iron, anhydrous or hydrated, and the sul- 

 phur of iron have co-operated. Indeed Ammonites are sometimes 

 transformed into Oligiste or Limonite. Many of these Ammonites, 

 partly ferruginous, are not less calcareous, like the formations in 

 which they are found ; often they are even converted into pyrites. 



To this explanation of the petrifaction of the remains of organized 

 bodies, by means of previous dissolution, it will perhaps be objected 

 that the carbonate of lime and silex, which are the most ordinary 

 agents in producing this phenomenon, are naturally insoluble in 

 water. But we know that the carbonate of lime dissolves in an ex- 

 cess of carbonic acid, particularly when aided by an increase of pres- 

 sure, and the bicarbonate of lime is met with in sea-water in con- 

 siderable proportion. With regard to silex, the action of alkalies, 

 the elevation of temperature, the gelatinous or nascent state induce its 

 dissolution, as is well known. The solubility which it acquires in these 

 circumstances may be conceived to admit of the formation of the zeo- 

 lithes and amygdaloids, which we so frequently meet with in the 

 neighbourhood of igneous rocks. Finally, in certain circumstances, 

 the silex is taken away from the rocks that contain it by the heat 

 of the water alone. It is to an effect of this nature that we must 

 refer the origin of these siliceous deposits, found in such great 

 quantities in Iceland, at the bottom of the Geysers. M. Dumas ad- 

 mits that, in these particular cases, the silex is dissolved by the re- 

 peated shock of the steam escaping from the warm springs. The 

 thermal springs of that country, indeed, contain a considerable 

 quantity of silex in a state of dissolution, caused by the double effect 

 of heat and alkalies. 



An increase of pressure is not perhaps without influence on the 

 solubility of this earth. We are tempted to believe this, on witness- 

 ing silex existing in a state of sohition in the greater part of subter- 

 ranean waters, such as the mineral springs applied to medical pur- 

 poses. Lastly, when we remark that a great number of vegetables 

 contain notable quantities of silex in their stalks, and in certain of 

 their members, and when we find silex in a dissolved state in the 

 waters of many rivers, we are led to believe that the greater part of 

 fresh waters contain small quantities of this substance. 



If we decline co admit the fact of a real dissolution of silex in the 

 waters which have petrified shells in geological times, it will be suf- 

 ficient, for the explanation of the phenomenon, that we admit its 

 gelatinous state. If it be necessary that silex should be in a state 

 of dissolution before it can produce rock-crystal, it is evident that, 

 by solidifying to the gelatinous state, it has produced the silices, and 

 particularly the agates and calcedonies. 



The preceding observations authorise us, therefore, to refer the 

 phenomenon of the petrifaction of organic remains, durincr the geo- 

 logical and historical epochs, to a mineral substitution brought about 



