On the relative position of the Serpentines 

 (OphiolitesJ, Diallage Rocks ( Euphotides ) , 

 Jasper, ^c. in some parts of the Apennines ; 

 hy Alexander Brongniart, Member of 

 the Royal Academy of Sciences, %c. %c. 



Read at the Royal Academy of Sciences, December 6, 1820. 

 (Annales des Mines for 1831). 



Introduction. 



IT is now very generally agreed that the end of positive 

 Geology is to be able to understand, as exactly and com- 

 pletely as possible, the nature and structure of the crust of 

 the globe, and to discover if general and constant laws have 

 governed this structure. 



The precise determination of the rocks, minerals, and 

 fossil organic bodies constituting the different portions of 

 this crust, and lastly that of their relative position, are the 

 principal and perhaps only means that can enable us to 

 acquire this knowledge, and lead us to the general, inte- 

 resting and useful results that it promises. All geologists 

 are agreed on these three propositions, and all endeavour to 

 discover by different means the order of superposition of 

 formations and the rocks composing them ; but all are not 

 yet equally agreed on the utility on determining previously, 

 and independently of every other consideration, the rocks 

 entering into the structure of formations. Some, without 

 altogether rejecting this mode of considering them, attach 



