STRATIGRAPHICAL GEOLOGY. 5l() 



fossiliferous Gault and unfossiliferous Lower Greensand. 

 D'Archiac's admirable work, Description geologique du Depart- 

 ment de rAisne, provided supplementary information about the 

 Cretaceous deposits in this part of France. The Upper or 

 White Chalk and the Greensand formations were shown to 

 be well developed, the lower horizons of the Cretaceous to be 

 absent. 



No less important was another work by the same author 

 on the middle group of the Cretaceous system (1839). 

 U'Archiac gave in this work a lucid exposition not only of 

 the Middle division but also of the whole Cretaceous series in 

 the marginal areas of the Paris basin, in Belgium, and the 

 neighbourhood of Aix. He compared the sequence of deposits 

 with the succession in England, Switzerland, and Germany, 

 and finally sub-divided the system as follows : 



{Upper horizons of Chalk (Maestricht, 

 Chalk Marl. 



[ Upper Greensand. 



Middle Group. \ Blue Marl (Gault). 



[ Lower Greensand. 



Neocomian (Marine facies). 



Lower Group. , , , /-r- u f Weald clay. 



- 



ees Hastings d. 



( Purbeck strata. 



While the greatest enthusiasm prevailed among French geo- 

 logists to elucidate the Cretaceous system, Germany had fallen 

 rather behind in this work. Friedrich Roemer and Hans 

 Geinitz were the leading contributors to the knowledge of the 

 German Cretaceous deposits. In 1836, Roemer had described 

 a marine deposit in the Hils basin under the name of "Hils 

 Clay," but had relegated the Hils clay, along with the Wealden 

 formation, to the Upper Jurassic horizon. In 1839, how- 

 ever, he demonstrated that the Hils clay was younger than the 

 Wealden formation, and possibly represented the Speeton clay 

 of England. The careful investigation of the fossils in the 

 Hils clay showed, according to Roemer, distinct affinities both 

 to Upper Jurassic and Cretaceous faunas. Two years later 

 Roemer published his important monograph of the Fossils 



