24 M. D'Omalius D'Halloy on 



cumscribed within the limits of the Paris basin ; and this is a 

 remarkable circumstance in the history of that formation; 

 at least I have not observed it elsewhere, although I have 

 made a point of examining the places, where, from minera- 

 logical resemblances, this formation has been said to exist, 

 and where I have recognized calcareous rocks older than 

 true chalk, possessing the texture and yellowish colour of 

 the Paris building stone.* 



I shall conclude this memoir with a few details on the 

 chalk formation, though, after the excellent description 

 contained in the Mineral Geography of the Environs of 

 Paris, there remains scarcely any thing to say on common 

 chalk ; the inferior strata of this great formation being in 

 contact with the southern part of the Paris basin, I conceive 

 that it will not be out of place to make its different modifi- 

 cations here known. 



Beds more or less differing from true chalk, by their 

 mineralogical character, by their chemical nature, and even 

 by peculiar fossils, separate this formation from the old ho- 

 rizontal limestone, + but approach true chalk by insensible 

 shades. The four following modifications may be distin- 

 guished in this series ; 1st, chalk with pale flints ; 2d, tuffa, 

 or coarse chalk often chloritous ; 3d, sand and sandstones 

 of the chalk, which are almost always mixed with limestone ; 

 4th, grey clay, generally marly, rarely plastic, and some- 

 times containing chlorite. The passages and alternations of 

 these different modifications one into the other, does not al- 

 low the order of superposition to be determined in a constant 

 manner. It may nevertheless be remarked, that the chalk 

 with pale flints is generally the newest, and that it precedes 



* This is the case with all the limestones existing beyond the chalk 

 belt that surrounds the Paris basin, whether towards Caen, Tours, 

 Bourges, or in Burgundy and Lorraine. 



+ It appears from a note to a paper inserted in the Annales des Mines 

 for 1822, p. 367, that the author included in this " bad name" the Al- 

 pine limestone (magnesian limestone), the lias, and the oolite formation; 

 in this case it apparently represents the latter, and perhaps also the lias. 

 (Trans.) 



