366 MINERALOGICAL CONSTITUENTS. 



respect particularly instructive: "De Maquenoise a Hirson* 

 la vallee ne presente que des fragments plus ou moms roules 

 des roches de transition que traverse le cours de la riviere. 

 En descendant a Etreaupont, on y trouve des calcaires juras- 

 siques et des silex de la craie, formations qui ont succede 

 aux roches anciennes. A Guise, le depot erratique .... est 

 compose de quartzites 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'a 

 celui de grains de sable. . . . Au dela les fragments de roches 

 anciennes diniinuent graduellement en volume et en noinbre." 

 At Paris the granitic debris brought down by the Yonne forms 

 a notable proportion of the gravel ; and at Precy, near Creil, 

 on the Oise, the fragments of the ancient rocks are abundant ; 

 but lower down the Seine, at Mantes, they are smaller and 

 less numerous, while at Eouen and Pont de 1'Arche I found 

 none, though a longer search would doubtless have shown 

 fragments of them. This case of the Oise is, however, inte- 

 resting, not only on account of the valuable evidence contained 

 in the above quotation ; but because, though the river flows, 

 as a glance at the map will show, immediately across and at 

 right-angles to the Somrne, yet none of the ancient rocks 

 which form the valley of the Oise have supplied any debris 

 to the valley of the Somme : and this, though the two rivers 

 are at one point within six miles of one another, and separated 

 by a ridge only eighty feet in height. 



The same division occurs between the Seine and the Loire : 

 " Bien que la ligne de partage des eaux de la Loire et de la 

 Seine, entre St. Amand (Mevre) et Artenay, au nord d' Orleans, 

 soit a peine sensible, aucun debris de roches venant du centre 

 de la France, par la vallee de la Loire n'est passe dans le 

 bassin de la Seine." f 



In the Vivarais near Auvergne, "Les depots diluvieus sont 



* D'Arcliiac, L c. p. 155. t Ibid. 1. c. p. 164. 



