

1823.] the principal Mountain Chains of Europe. 357 



but the best opportunities of studying the whole formation are 

 afforded by the eastern coast of Dorsetshire (the Isle of Purbeck), 

 by the Isle of Wight, and more especially by the great denuda- 

 tion of the beds beneath the chalk in the south-eastern counties, 

 including the Weald of Kent, Surrey, and Sussex, where all the 

 beds above enumerated are displayed on the fullest scale. In 

 the other British Islands, this formation only occurs in the north- 

 east of Ireland, where it may be seen near Belfast supporting 

 the chalk which underlies the great basaltic area of that district. 



(B.) France. 



In France these formations have been observed among those 

 which circle round both the northern basin of the Seine and the 

 south-western basin of the Garonne. 



1. They range beneath the escarpment of the chalk hills sur- 

 rounding the denudation of Boulogne, a continuation of that 

 just mentioned as occupying the south-eastern counties of 

 England. 



2. They skirt round the exterior of the chalky zone bounding 

 the basin of Paris, forming a broad sandy tract. Mr. de la 

 Beche has distinctly described the commencement of these 

 chains from the channel near the mouth of the Seine, and illus- 

 trated it in his excellent sections published in the Geological 

 Transactions for 1822. 



M. Omalius d'Halloy has described the series under the title 

 of the lower chalk, which seems very unfortunately chosen, since 

 it is only mineralogically applicable to a very small part of it 

 (that corresponding to our chalk marl), and has led to much 

 confusion both as to the description of the chalk formation itself, 

 and its constituent fossils. This author notices the following 

 subdivisions: 1. Chalk 5 sometimes of a coarser texture, occa- 

 sionally mixed with clay, sand, and chlorite, containing pale 

 flints abundantly. 2. Tuffeau ; coarse sandy chalk mixed with 

 chlorite. 3. Sands and sandstones ; often mixed with calcare- 

 ous matter. 4. Greyish clay ; commonly of a marly character, 

 sometimes mixed with chlorite : the passages of these modifica- 

 tions into one another, and their alternations prevent the decided 

 determination of their order of superposition, further than the 

 assigning the highest position in the series to No. 1. 



The green sand occurs beneath the chalk at Valenciennes ; it 

 is there called Turtia. 



3. In the basin of the Garonne this formation has been parti- 

 cularly observed by M. Boue along its northern border, stretch- 

 ing from the Island of Aix near la Rochelle, to Perigord. 



Humboldt has extracted from an unedited memoir of M. Fleu- 

 riau de Bellevue, some very interesting particulars concerning a 

 arge deposit of lignites connected with this formation in the 

 vicinity of the Isle of Aix. They consist of dicotyledonous 

 plants, partly petrified, partly bituminized, and sometimes in the 

 state of jet ; they are generally compressed, and lie sometimes 



