SS3 Proceedings of Philosophical Societies. [Nov. 



M. de TIalloy has recognized three very distinct modifications. It . 

 is itself surrounded, except towards the sea, by a compact lime- 

 stone older than it, which forms a great part of Berry, of Burgundy, 

 and of Lorraine as far as Vosgcs, and which appears beyond the 

 Black Forest in Franconia and Hesse. The formations of the system 

 of Paris extend over this chalk divers ramifications, and the agri- 

 culture, the industry, and all the resources of each place, are often 

 determined by the geological constitution of the soil. M. de Halloy 

 has not displayed less courage than sagacity in collecting the mate- 

 rials for his memoir ; for he traversed the whole country on foot, 

 visiting the most inaccessible places when he could hope for any 

 information, and being neither deterred by bad weather nor bad 

 roads. 



M. Brogniart, corresponding member of the Institute, has like- 

 wise visited a part of France no less interesting for geology, that 

 which forms at present the department of La Manche ; and M. de 

 Halloy, who visited it after him, has confirmed and completed his 

 observations. From the description which M. Brogniart gives of 

 the rocks of this councry, and of their mutual position, it follows 

 that what was considered as granite belongs to another species of 

 rock, called syenite by Werner, and characterized by the horn- 

 blende which enters into its composition, as well as by its much 

 more recent formation, than true granite. The syenite of the 

 Manche reposes on slate, and other rocks posterior to granite. It 

 appears even that in certain places it covers a lime-stone containing 

 the remains of organized bodies ; a fact analogous to what Von 

 Buch observed in Norway, and from which we may conclude that 

 there have been precipitations of crystallized rocks after the mani- 

 festation of life in the waters which anciently enveloped the globe. 

 M. Brogniart, who is employed in drawing up a general treatise 

 on geology, has presented the plan according to which he proposes 

 to arrange the rocks ; that is to say, those aggregations of minerals 

 which compose the crust of the globe such as we know it. Applying 

 the principles at present acknowledged by all naturalists, he wishes 

 the basis and the details of his method to repose upon characters 

 taken from the rocks themselves, and he rejects all those taken 

 from their mutual position on the globe, which belongs to their 

 natural history, but not to their systematic arrangement. He 

 separates from the rocks, and unites to simple minerals, the mineral 

 bodies which appear simple to the naked eye, and the heterogeneous 

 nature of which appear only by washing and other operations, 

 which, though they cannot be called chemical analyses, yet alter 

 the appearance and the tissue of these matters. Such'are slate, 

 clay, he. The rocks thus reduced, or the mixed rocks, as M. 

 Brogniart calls them, arc subdivided into crystallized and aggre- 

 gated. The first have their parts either nearly in equal pioportiun--, 

 or one of the constituents predominates over the others. In the 

 first case the genera are established according to the essential con- 

 stituents ; that is, those which arc constantly found. In the second 

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