280 M. D'Halloy on the Geographical Extent [ApriI, 



this hypothesis we should find among them the debris of the 

 different rocks of which the neighbouring countries are com- 

 posed, as is the case with the alluvial deposit of the Loire, where 

 we can trace even among the finest sands the mica and felspar 

 of the granite of Auvergne, In no part does a country exist so 

 exclusively composed of quartz that the destruction of its rocks 

 could give rise to the sands we are now considering ; and the 

 supposition of such a country being entirely destroyed, or con- 

 cealed, is much more at variance with what we know of the 

 operations of nature, than the opinion that these sands have been 

 formed as we now see them, in the same manner as the different 

 sandy strata, of which the local formation is actually demon- 

 strated as well by their alternation with other rocks as by the 

 fossils they contain. 



The first idea that presents itself on this latter hypothesis is, 

 to consider the sands of the northern part of the Sologne as 

 belonging to the formation of the older chalk, as well as those of 

 the southern part of the same district, of Tourraine, Perche, &c. 

 The existence in these latter of coarse grained beds, resembling 

 the sands between the Loire and the Saudre, adds strength to 

 this opinion. On the other hand, the presence of these sands 

 on the freshwater limestone on the banks of the Loire and in the 

 Gatinais, the occurrence of detached patches of similar sands 

 on the same limestone in other places nearer to Paris, as at 

 Etampes, Rambouillet, &c. and, lastly, certain points of agree- 

 ment between them and the buhrstone formation, might suggest 

 the idea that they are the latest bed of the second freshwater 

 formation of the Paris basin, as has been already supposed by 

 Messrs. Cuvier and Brongniart in respect to the sands found on 

 the summits of the hills of Longjumeau.* 



I confess that I am still at a loss to decide between these two 

 opinions, and that in excluding the country between the Loire 

 and the Saudre from the basin of Paris, I have been determined, 

 in the absence of geological indications, by considering it as a 

 question of physical geography, and on that account think it 

 rifht not to separate the parts of a region so naturally united as 

 the Sologne. It is, however, proper to remark on this subject, 

 that on the hypothesis that all the sands of this country belong 

 to the older chalk, we may well conceive the possibility of then- 

 extension over the freshwater beds ; for this deposit of inadherent 

 matter being situated precisely at the part where the waters pass 

 as they descend from the mountains of Auvergne, must have 

 been more disturbed by the waters than those situated differently, 

 and some great catastrophes, such, for instance, as that which 

 has overwhelmed the animals whose remains are found in the 

 alluvial marl and gravel, may have sufficed to throw a part of 

 these sands on the slightly elevated country of the freshwater 



* Mineralogical Geography of the Eimrons of Paris, p. 65. 



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