292 MINERALOGY OP THE SOUTH OP ENGLAND. 



Extended plains, which lie high above the present river 

 courses, and then they consist of gravel and sand, it is 

 difficult to distinguish the alluvial mud, situated at a dis- 

 tance from the valleys, from the fresh water formations, 

 and it even, in some places, seems to pass into it. It ap- 

 pears, however, to be older than that of the valleys. 



GENERAL OBSERVATIONS. 



The eleven different formations now described are 

 considered by Cuvier and Brongniartto be partly of ma- 

 rine, partly of fresh water origin, these distinctions de- 

 pending on their containing salt or fresh water petrifac- 

 tions. On this principle the formations are viewed as 

 follows : 



Formation. 



1. Chalk. 



2. Plastic clay, <kc. 



3. Coarse marine limestone. 



4. Siliceous limestone without shells. 



5. a. Marl at the bottom of the 

 gypsum formation. 



b. The layers of marl, gypsum,^ 

 and adhesive slate above the pre- ^ 

 ceding. 



c. The great bed of gypsum. 



6. Marine marl above the great > 

 bed of gypsum. 3 



T. Sandstone and sand without shell. 



8. Marine sandstone and sand. 



9. Millstone without shells. 



10. Flint siliceous limestone. 



Origin. 

 Marine 

 Fresh water. 

 Marine. 

 Not determined. 



Fresh water. 



Marine. 



Fresh water. 



Marine. 



j 



Not determined, 



Marine. 



Not determined 



Fresh water, 



