144 Proceedings of ihe 



of the rock or stratum itself and the minerals which accompany it 

 this is the mineralogical characteristic ; and 3d, the organic remains 

 contained in it this is the zoological or organic characteristic. Of 

 these, the second is the least certain. By chalk deposits, in a geological 

 sense, therefore, we mean not merely those composed of white chalk, 

 but such as occupy the same position in the beds of the globe as 

 chalk usually occupies ; which contain the same species of organic 

 remains ; but which may or may not present the same mineralogical 

 characters. Hence chalk formations (terrains cretaces) may be black 

 and compact, yellow and compact, in masses or in strata, with or 

 without silex, and even wholly composed of sand and freestone, with- 

 out containing any mineralogical chalk, and even scarcely any carbo- 

 nate of lime. Hence it is that it has been hitherto supposed that there 

 were no chalk formatious in the south of France and the Pyrenees. 



M. Dufresnoy, by his observations, has established three series of 

 facts : 1st. He has recognized these formations in parts of France 

 and Spain in which their existence was hitherto unknown ; 2dly, he 

 has shown that they contain mineral masses which were supposed 

 wholly foreign to them ; and, 3dly, he has, on the one hand, aug- 

 mented the number of their zoological characteristics, and, on the 

 other, has diminished the negative importance hitherto attached to the 

 absence of certain shells. In the south of France these formations 

 have been recognized as forming a subterranean valley, the northern 

 and southern borders of which show themselves by hillocks or 

 mounds of earth, separated from each other, but tracing, by their 

 disposition, two zones from east to west. The northern xone com- 

 mences in the south of La Vendee, near Rochefort and Royant, and 

 extends to the foot of the maritime Alps. The southern zone rests 

 on the northern declivity of the Pyrenees, commencing from the 

 eastern extremity of the Corbieres, and extending as far as Bayonne 

 in a narrow band. At Bayonne it becomes wider, and, entering 

 Spain, extends to Cardonne. The valley inclosed by these zones is 

 almost entirely filled with terrain tertiaire arid alluvial earths. M. 

 Dufresnoy remarks that, by a general and almost regular elevation 

 of the granite chain of the Pyrenees, chalk formations have 

 been carried to a great height, acquiring a compact texture and 

 a black colour : Mont Perdu, a summit 3500 metres above the level of 

 the sea, belongs to this class of chalk formations. Another elevation, 

 that of the ophites, has also, though in a much smaller degree, 

 deranged the horizontal position of these deposits ; but as the 

 eruptions of ophite, to which this derangement is probably owing, 

 have been much less abundant on the southern than the northern 

 base of the Pyrenean chain, the chalky formations have been much 

 less deranged from their position on the Spanish than on the French 

 side. The proofs which M. Dufresnoy gives of the nature of these 

 formations are quite satisfactory : indeed they have even proved the 

 existence in France of a group of chalk deposits (the Weald group), 

 which had previously been only observed in England. The chalk 



