Observations on the retrij'actiun of ^'/(e/la, >^r. 51 



In fact, the waters which filled the basin of the seas of geological 

 times, not only occupied very large spaces, but they also possessed a 

 more energetic dissolving property, if we may judge from the great 

 quantity of substances which they have deposited on the surface of for- 

 mations. A similar comparison between the fresh waters of the old 

 world, and those which now fill the lakes and lowest points of con- 

 tinents, will lead to the same conclusion. Indeed, it does not ap- 

 pear that existing fresh waters can produce such considerable de- 

 posits as those which have been left by the rivers and lakes of the 

 old woi'ld. 



The important part which carbonate of lime and silex have per- 

 formed in the phenomenon of petrifaction, appears from a simple 

 examination of facts. The greater part of geological petrifactions 

 have been produced by means of carbonate of lime. This pheno- 

 menon is always more complete when the waters among which it 

 takes place contain this salt in abundance. When the gypseous 

 formations contain organic remains, which i-arely happens in regai'd 

 to the mollusca, they are found to be in a state of incomplete petri- 

 faction, as may readily be seen by examining the bones found in 

 such places. It is the same with the most part of the arenaceous for- 

 mations and deposits of clay, where the shells preserve their distinct 

 appearance as well as in the gypseous formations. It is, in i'act, 

 among marls and sands that the remains of the life of geological 

 times are best preserved and most frequently found. 



After the carbonate of lime, silex is the most frequent agent of 

 petrifaction ; it is superior even to the cai'bonate of lime in the 

 fidelity and delicacy with which it reproduces the finest lineaments 

 of organic remains. 



Besides, certain peculiarities of these organized bodies appear to 

 have been not without influence on the siliceous pscudo-morphoses. 

 Ihus, the parts of bodies of sufficient consistency to preserve their 

 form during the time necessary for petrifaction, are almost always 

 in the calcareous state, and those of less consistency have passed 

 into the siliceous state. We often see the ligaments of Grypheac 

 changed into silex, although the head be petrified with calcareous 

 matter. The greater part of the fossil Alcyons and Sponges are 

 almost always transformed into silex. In like manner, the nuclei 

 of these shells are more frequently metamorphosed into silex than 

 their heails. The Ananchites and the other Echinides of the green 

 sandstone, whose head is almost always calcareous, have a siliceous 

 nucleus in the intei'ior, which often fills the whole space. One would 

 say, that the animal matter has sometimes issued from it as if squeezed 

 out by mechanical pressure. Finally, the siliceous zoophytes are 

 often found disseminated throughout calcareous rocks, which seems 

 to indicate a kind of electrive attraction of the animal matter for 

 silex. 



Carbonate of hnie and silex are not the only substances which 



