290 MINERALOGICAL CONTENTS 



the coarse gravel, but only here and there, where quieter 

 conditions, indicated by a seam of finer materials, have pre- 

 served them from destruction. Here, therefore, we have a 

 conclusive answer to the suggestion that the gravel may 

 have been heaped up to its present height by a sudden ir- 

 ruption of the sea. In that case, we should find some marine 

 remains ; but as we do not, as all the fossils belong to animals 

 which live on the land, or inhabit fresh waters, it is at once 

 evident that this stratum, not being subaerial, must be a 

 freshwater deposit. 



But the gravel itself tells us even more than this : the 

 river Somme flows through a country in which there are no 

 rocks older than the chalk, and the gravel in its valley con- 

 sists entirely of chalk flints and tertiary debris.* The Seine, 

 on the other hand, receives tributaries which drain other 

 formations. In the valley of the Yonne we find fragments of 

 the crystalline rocks brought froin the Morvan,f The Aube 

 runs through cretaceous and Jurassic strata, and the gravels 

 along its valley are entirely composed of materials derived 

 from these formations. The valley of the Oise is in this 

 respect particularly instructive : " De Maquenoise Hirson J 

 la vallee ne presente que des fragments plus ou moins 

 roules des roches de transition que traverse le cours de 

 la riviere. En descendant Etreaupont, on y trouve des 

 calcaires jurassiques et des silex de la craie, formations 

 qui ont succede aux roches anciennes. A Guise, le depot 

 erratique , .... cat compose* v de quart zites et de schistes 

 de transition de quelques gres plus recents, de silex de 

 la craie, et surtout de quartz laiteux, dont-le volume varie 

 depuis celui de la tete jusqu'd celui de grains de sable .... 

 Au dela les fragments de roches anciennes diminuent gra- 

 duelleinent en volume et en nombre." At Paris the 



* Buteui, I.e. p. 98. f D'Archiac, Progres de la GSologie, p. 163. 



J D'Archiac, I.e. p. 155. 



