278 M. D'Halloy on the Geographical Extent [April, 



these regions commonly known by the name of Perche, is a 

 woody country furrowed by numerous valleys which extends 

 from the vicinity of l'Aigle towards Montdoubleau ; the other 

 comprehends the dry elevated plains between the Sarthe and 

 the Loire, and extends a little to the north of the first of these 

 rivers. 



These sands might, at first sight, be taken for an alluvial de- 

 posit, and the rather, because from the want of adherence of 

 their particles, the upper parts have been disturbed by the waters, 

 and are often mixed with rolled pebbles ; but on a more careful 

 study, we soon become convinced that they belong to the forma- 

 tion of the older chalk. We observe in fact that the lower parts 

 of the chalk gradually become coarser and contain more sand, 

 and that then the siliceous nodules often pass into the state of 

 calcareous sandstones. We also see that this sandy chalk alter- 

 nates with regular beds of sand and sandstone, which contain 

 the fossils characteristic of the ancient chalk, and we can dis- 

 tinctly recognize in several places that the principal mass of the 

 sandy strata dips under that of the chalk. 



These sands and sandstones generally contain calcareous 

 matter, and sometimes chlorite ; but some of them are quite 

 pure : the greater part are of a fine equal grain, others of a coarse 

 irregular texture. Their colour is commonly yellowish, some- 

 times whitish, rarely blue, reddish, or ferruginous. This last 

 colour belongs principally to some deposits of sandstone passing 

 into pudding-stone, called in the country roussard, and which are 

 found buried in the middle of the sands. In general, organized 

 bodies are not found in beds composed of pure quartz ; but they 

 are often very abundant in those which contain calcareous 

 matter ; the most common are the orbicidar gryphoea, and some 

 species of ostreae. The ammonites here begin to appear, or to 

 speak with more exactness, these strata appear to have been 

 formed at the latest period of the existence of these animals ; 

 but we shall probably find, when the species of this genus are 

 better known, that the ammonites of these beds are different from 

 those of the alpine limestone. Remains of fish and impressions 

 of vegetables * are also found in these beds. 



The ancient province of Touraine lies to the south of this sandy 

 district, and extends to the lias which is found to the south of 

 Chatellerault and Chatillon sur Indre. The soil is of coarse 

 grained chalk. This substance, known in the western depart- 

 ments under the name of tuffeau, is sometimes tender and 

 friable, and at others is of sufficient hardness for building ; its 



* This last observation was made by M. Maulay, a naturalist, residing at 

 Mans. I should here remark that I do not consider all the sands which cover the 

 elevated plains between the Sarthe and the Loire, to belong to the chalk formation; 

 for the occasional presence of some thin beds of buhrstones and the existence of 

 a small deposit of freshwater limestone, near Mans, renders it probable that there 

 are some superficial deposits uf freshwater sandstone ; hut I have not had the means 

 of ascertaining this fact. 



