( 50 ) 



Observations on the Pettif action of Shells in the Mediterranean. 



By MM. Marcel de Serres and L. Figuier. 



All the researches of modern geology seem to prove that nothing- 

 is changed in the order of nature, and that the same causes which 

 operated in the first ages of the world, are still influencing the oc- 

 currences that take place under our own eyes. Certain facts, how- 

 ever, have hitherto appeared not to be referable to this common 

 origin ; and the petrifaction of organic remains, in the midst of 

 geological formations, is daily adduced as one of the most weighty 

 arguments against this general law. 



Few persons, indeed, will be ready to admit what, however, is an 

 indisputable fact, that there are now forming, in the bosom of seas, 

 petrifactions which, in the double respect of chemical composition 

 and mode of petrifaction, are altogether analogous to those which 

 are formed in the bed of the ancient sea. Tiie object of this me- 

 moir is to demonstrate this general fact, and to study the pheno- 

 mena by means of which it is brought about. 



We hope to prove, at the same time, that the sandstones contain- 

 ing the molluscous remains which cover, as is well known, spaces of 

 such vast extent in the tertiary formations, have their analogues in 

 the shelly rocks of recent formation, which are formed, in our own 

 day, in the middle of the Mediterranean. 



I. On the Mode in which Organic Bodies became Petrified in 

 Historical and Geological Eras. 



If we reason according to the facts which occur in our own day, 

 certain conditions seem necessary to produce the petrifaction of 

 organised bodies, or, in other words, these petrifactions are not ob- 

 served but where these conditions are united. We may admit, with- 

 out deviating too much from probability, that the same circum- 

 stances vfQYQ necessary to produce this phenomenon during the geo- 

 logical epochs. 



In order that organic remains may become petrified, that is to 

 say, in order that tlie organic matter they contain may be replaced 

 by a mineral substance which preserves their forms and most deli- 

 cate lineaments, it is necessary, in our opinion, 1st, that these remains 

 should be sunk in great masses of water ; 2dly, that the watei's 

 should contain, in certain abundance, calcareous or siliceous salts. 



It is easy to conceive, that the first of these conditions must have 

 been constantly present daring geological times, as well in respect 

 to the organised species deposited in seas, as in regard to those 

 existing in fresh waters. To be convinced of this, it is sufficient 

 to compare the extent occupied by seas at periods when thei'e was 

 no human witness, with that which thoy now occupy. 



