DOMAIN X 



the upper and( lower: part of the bed of this 

 stone, and are found thus contiguous, on the one 

 side to ,thc white chalky stone, and on the other 

 to the silicicalce. There are also scattered here 

 and; there, in the body of the chalky stone, some* 

 small flints, and some small silicicalces, which 

 are not fragments, but pieces formed in the spots 

 they occupy. 



" These observations and experiments appear 

 to me to prove that these intermediate kinds we 

 have sometimes represented as passages from one 

 kind-' to, another, or as limestones half metamor- 

 phosed into flint, are often only mechanical 

 mixtures of one kind with another. We here 

 see that the calcareous earth has preserved in 

 this petrosilex all its solubility in acids; and 

 when we extract it from the mixture, what re- 

 mains separated from the dissolvent, is still re- 

 fractory like pure silex. 



" I shall also draw an example from- this stone 

 of the insufficiency of the external characters of 

 a rock to determine its nature, and even only to 

 decide whether it be simple or compound. In-, 

 deed in the silicicalce, the calcareous parts are 

 not combined with the siliceous, since the nitrous 

 acid extracts them with effervescence without 

 destroying the aggregation of the stone. They 

 are then only interposed between the siliceous 



