162 M. Broxgniart on the posilton 



very little importance to it; others, confounding it with the 

 state of relative positions (positions respectives), desire that 

 mineralogical descriptions should constantly accompany those 

 of the formations. We have for a long time exerted our efforts 

 to prove that these two subjects ought to be separately treated, 

 that their confusion is necessarily carried into the science 

 and retards the progress of it ; we have with pleasure seen 

 these principles admitted by many foreign and French geolo- 

 gists, and it is to be remarked that those even who do not 

 desire explicitly to recognise them are compelled to submit 

 to them, as it were in spite of themselves, in their geological 

 observations. Those that I am about to bring forward will 

 again prove, at least I venture the hope, the necessity of 

 this distinction. It will be seen that it is very difficult, often 

 even impossible, to be able to determine the relative po- 

 sition of formations composed of particular rocks and fossil 

 organic bodies, if the differences of these rocks and fossils 

 have not previously been made known with precision. 



Among the rocks whose relative position in the crust of 

 the globe are either obscure or little known, are reckoned 

 certain ophiolites* (ophiolites), or rocks with a serpentine 

 base, diallage rocks, and even jaspers ; notwithstanding the 

 labours and numerous .travels for some time undertaken by 

 celebrated geologists, the knowledge of the position of 

 these rocks has remained either incomplete or uncertain ; 

 and I even venture to say, that in many cases, a very false 

 idea has been formed of it; this state of uncertainty was 



* 



principally owing to three causes : 



1st. To the rarity of one of these rocks, the Jasper. 



* I have given the characters of these composed rocks and those of 

 their varieties in my essay on the mineralogical classification of mixed 

 rocks, inserted in the Journal des Mines, vol. xxxiv. No. 199, July 1813. 

 It is found translated into German, in the work entitled, " Taschenbuch 

 fiir die gesammte Mineralogie ; " by H. C. Leonhard, 9th year, page 

 378; and into Italian, in M. Moretti's work: " Classificazione delle 

 Rocce. &Milano, 1814." M. de Bonnard has inserted the whole with 

 some modifications and additions, which I am disposed to admit, in the 

 2d edition of the " Nouveau Dictionnaire d'histoire naturellsj" at the 

 word — roche. 



