Institute of France. &.> 



soil. M. de Halloy visited all these places on foot, and was not 

 disheartened either by the badness of the weather or of the 

 roads. 



M. Brongniart, a corresponding member of the Class, has 

 visited a part of France equally interesting in a geological point 

 of view, viz. the department of La Manche : and M. de Halloy, 

 who followed his steps, confirmed and completed part of his 

 ob>>ervations. From the desevipUon given by ^I. Brongniart of 

 the rocks of this country, and of their nmtual position, it results, 

 that what were denominated as granites properly so called, be- 

 longed to that other kind' of rock called syenite by Werner, 

 and characterized by the amphibole which enters into its com- 

 position, as well as from its formation being much more recent 

 than that of the true granite. These systems of La Manche 

 repose on schists and other rocks long posterior in formation to 

 the granite: it even appears that in certain places they have 

 under them calcareous matter containing fragments of organised 

 bodies ; a fact which would be analogous to that observed iu 

 Nonvay by M. de Bncli, and from which we might conclude 

 that there were also ])recipitations of crystallized rocks, after 

 manifestation of life in the waters which anciently enveloped 

 the globe. 



I\L Brongniart, who is occupied with a general Treatise of 

 Geology, has presented the plan according to which he pro- 

 poses to distribute tlie rocks, /. e. those aggregations of minerals 

 which coaijiose the crust of the present globe. Apj)iving the 

 ))rinciples at present recognised by all naturalists, he wishes the 

 bases and the details of the whole of liis method to rest on 

 characters taken from the rocks themselves; and he rejects all 

 those which might be taken from their natural position on the 

 globe, which belongs to tlicir history, but not to their systematic 

 division : lie separates the rocks, and leaves among the simple 

 minerals the mineral substances which appear to be simple to 

 the naked eye, and the heterogeneousness of which is manifested 

 only I)y wasliiiig and other operations, which, without being re- 

 garded as chemical analysis, nevertheless alter the appearance 

 and texture of tliese substances ; such are the schists, argil, &:c. 

 Rocks so reduced, or, as M. Brongniart expresses himself, the 

 mixed rwki, are subdivided into crystallized and aggregated : 

 the former have their parts in proportions nearly equal, or rather 

 one of tliese parts prevails over the others : in the former case the 

 genera are established according to the essential substances, i.e. 

 those wliich are constantly there: in the latter case, according 

 to the base, ?'. e. the prevailing substance: and in both cases 

 the number of component substances, and the structure resulting 

 from their mode of union, serve to distinguish the species. 

 D 4 The 



