INTRODUCTION. IO5 



maps, and sections. The stratigraphical succession was 

 slightly changed ; eleven sub-divisions were recognised instead 

 of nine, the millstone quartz in No. 8, and the marine oyster 

 beds in No 4, being erected into independent sub-divisions. 



Upon the basis of their measurements of the thickness of 

 individual deposits, Brongniart and Cuvier were able to arrive 

 at definite conclusions regarding the configuration of the chalk 

 surface before the deposition of the plastic clay. They demon- 

 strated that the clay had been deposited upon an irregular^ 

 surface of pre-Tertiary hills and valleys, and that, owing to the 

 inequalities of the base of deposit, neither the clay nor the 

 succeeding coarse limestone series extended over the whole 

 area as connected layers. After the deposition of the coarse 

 limestone, the sea withdrew, and the Paris area then became 

 a fresh-water basin in which calcareous, gypsiferous, argil- 

 laceous and marly sediments successively accumulated. The 

 gypsiferous strata were thickest in the middle of the basin, but 

 neither they nor the fresh-water sediments were smooth layers. 

 It was only when the sea once more had ingress and brought 

 into the basin immense quantities of sand that an even surface 

 of deposit was attained. Again the sea retreated, and the area 

 became one of marshes and lakes in which the younger cal- 

 careous and siliceous deposits gathered; as the area continued 

 to emerge the surface was eroded, and valley depressions and 

 uplands took shape which were quite independent of the pre- 

 Tertiary configuration. 



The importance of this work for geology will be realised 

 when it is remembered that with the exception of formations 

 i and 9, all other formations in Brongniart and Cuvier's Table 

 were unknown in Werner's system of the rock-succession 

 (p. 58). Afterwards it was demonstrated that many of the 

 fossils of the Paris basin agreed with the fossils in the deposits 

 near Verona which Arduino had termed Tertiary deposits. And 

 the series was then incorporated in the chronological succession 

 of the rocks as the Tertiary formations. 



This was also the first French work which adopted the - 

 method introduced by William Smith in England ten years 

 previously, of determining the respective ages of the rocks by 

 means of the fossils contained in them. And in this sense the 

 work had a revolutionary effect on French geology. 



In a later publication Brongniart extended his observations 

 to the fresh-water deposits of other neighbourhoods Orleans, 



