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



Article VII. 



Memoir on the Geographical Extent of the Strata of the Envi- 

 rons of Paris. By J. J. Omalius D'Halloy. 



{Concluded from p. 95.) 



I shall conclude this memoir with some details on the chalk 

 formation, although, after the excellent description contained in 

 the Mineralogical Geography of the Environs of Paris, nothing 

 more is required on the subject of the common chalk ; but the 

 lower beds of this great deposit being in contact with the south- 

 ern part of the basin of Paris, I think that an account of their 

 modifications may, with propriety, be here introduced. 



Beds differing, more or less, from the common chalk in their 

 mineralogical characters, as well as in their chemical nature, and 

 in the presence of particular fossils, separate the strata of the 

 Paris basin from the older horizontal limestone, or lias ; but are 

 connected by insensible gradations with the true chalk. Among 

 these shades of gradation the four following modifications may 

 be distinguished: 1. Chalk with pale flints; 2. The tuffeau, 

 or coarse chalk, frequently mixed with chlorite ; 3. The sands 

 and sandstones of the chalk formation which are generally mixed 

 with calcareous matter; 4. Greyish clay, commonly of a marly 

 character, seldom plastic, and sometimes mixed with chlorite. 

 The passages of these different modifications into each other, and 

 their alternations, prevent the decided determination of their order 

 of superposition. We may, however, remark, that the chalk 

 with pale flints is the most recent, and that it precedes the 

 common chalk with dark coloured flints, from which its separa- 

 tion is not always to be distinguished ; that, on the other hand, 

 the clay beds are the oldest of the formation, and that there is 

 even a part of these beds that belongs rather to the lias than to 

 the older chalk. 



Fossils abound in these different beds ; some, such as the 

 echini, are the same as those of the common chalk ; others, as the 

 ammonites, are similar to those of the alpine limestone ; some, 

 namely, the belemnites, terebratulae, &c. are common to them, to 

 the chalk, and to the alpine limestone ; those which may be consi- 

 dered as characteristic, as well by their abundance in these beds 

 as by their rarity, or, perhaps, their total absence in other strata, 

 are the orbicular gryphite, and a large shell referable to the genus 

 spondylus. 



The large chalk basin which extends like a gulph into the 

 north west of France, presents these modifications of the older 

 chalk throughout its circuit, except on the side of the English 

 channel, where the common chalk reaches to the *ea coast. 

 Every where else we may generally recognize the lour series just 



