3^ M. D'Omalius D'Halloy o«fl . 



chalk. The limestone beds predominate in the Paris basin. 

 Lastly, in the plains watered by the Saone, the Rhone, the 

 Po, the Aar, the Danube, &c., we are struck with the quan- 

 tity of rounded fragments which are buried in the sandy and 

 clayey deposites. 



The Tolcanic rocks have too different an origin, and their 

 existence belongs to phenomena which are too remarkable, 

 not to be noticed on a geological map, however small the 

 space occupied by them may be ; but the establishment of 

 this group would present many difficulties, if I knew not 

 where to stop among those rocks which bear a greater or less 

 resemblance to the products of existing volcanoes. But now 

 that the researches of M. Beudant have thrown new light 

 on this branch of geology, I considered that I might class 

 the products of modern volcanoes with the two systems 

 known by the names of trachytic and basaltic rocks. 



It should be remarked that this group does not correspond, 

 like those which precede, to a fixed epoch of formation ; for 

 while volcanic rocks are now forming, there are trachytes 

 and basalts of more ancient formations than some rocks 

 classed in the preceding groups. 



The exposition of these divisions would certainly have 

 made the imperfection of our geological nomenclature felt, if 

 its defects were not generally admitted ; yet we have not 

 effectually endeavoured to correct it. It must in fact be 

 confessed that it is a task that would require considerable 

 knowledge in the person who should undertake it, I there- 

 tore do not pretend to do so ; but as it has fallen to my lot 

 to establish some new divisions, I considered that they should 

 receive new names. 



The following are the names that I propose to give the 

 five groups into which I have divided the secondary rocks. 



I shall name the first penean rocks (terrains peneens), 

 which is but the translation of todt-liegende, and which may 

 besides be considered as expressing the circumstance that 

 the most characteristic beds are poor in organic remains. 



The second group will be named ammonean rocks (terrains 

 ammoneens) because all the systems of which they are com- 



