298 M. D'Omalius D'Halloy OM a 



divisions established by authors, should yet preserve the 

 most essential sections, and should agree with the develope- 

 ment of the diflFerent rocks in the countries I wished to re- 

 present : it will easily be understood that if some rocks fine 

 off so that their existence cannot be separately marked on 

 the map, and that these rocks possess general relations which 

 distinguish them from other groups, it would be better to 

 represent them by a common sign, rather than to undertake 

 a distinction that could only be adopted in a few instances. 



Guided by these considerations, and after numerous trials, 

 I have been led to tlie system which has served as a basis to 

 the map of which I hazard the publication.* I am far from 

 considering this system to be without objections, perfection 

 will never be attained in a first sketch, if perfection could be 

 found in the works of men. 



I shall not now undertake a particular examination of the 

 different groups of rocks traced on the map ; but I shall 

 shew the principles that have guided me iu the formation of 

 these groups. 



The old division of rocks into primitive and secondary., 

 i. e. anterior and posterior to the existence of organized 

 beings, can no longer accord with the intimate union that 

 has been remarked between some primitive rocks, and beds 

 containing organic remains ; the celebrated school of Frey- 

 berg introduced an intermediate class in which to place these 

 last beds. Since that time, new observations have proved 

 that these intermediate rocks, instead of being constantly 

 posterior to all rocks tliat had the general characters assigned 

 to primitive formations, occur included between crystallized 

 rocks, which do not contain any organic remains. 



* The work which I have undertaken conjointly with M. Coquebert 

 de Monbret, containing many more details than could be represented in 

 the small map joined to this memoir, we conceived it might be useful to 

 publish it on a larger scale; we have in consequence made it the subject 

 of another and more extensive map, on which M. de Monbret has more- 

 over added various agricultural information, such as the limits of the 

 countries_jn which the vine, olive, orange, &c. are cultivated. 



