Mr. De la Becue on the Geologi/ of Part of France. 83 



which the forest stands is composed of both. From Lison towards Isigny 

 there is much red marl mixed with the gravel beds, and the same rock is seen 

 also at the bridge over the Vire near Isigny. 



Gravel formed of rounded quartz pebbles is in Calvados the principal mem- 

 ber of this formation : it is associated with sand_, generally whitish, seldom 

 red, and occasionally with red marl ; the pebbles are rarely cemented, as in 

 the new red sandstone conglomerate of the coast of Devonshire. 



In the department of La Manche the new red sandstone formation occurs 

 abundantly in the neighbourhood, and to the east and south-east of Carentan; 

 the road from St. Lo to the latter place affording a good section. It rests, 

 near St. Jean Day, upon argillaceous slate ; thence to Carentan the country 

 is composed of red marl and red sandstone, mixed with the usual blue and 

 white Simla. 



The gravel beds so common in the department of Calvados are more rare 

 in this part of the department of La Manche. 



Near Brevans and Cats, on the point of land between Isigny and Carentan, 

 the new red sandstone is yellowish, mixed with red and grey, and is tolerably 

 compact. From Carentan to Sainteny, and towards Beaute, beds of gravel 

 and sand occur ; but the sand is seldom red. 



The gravel beds above mentioned occur between Lestre and Montebourg. 

 From Valognes, on the road to St. Vaast, the gravel beds, red marl, and 

 red sandstone, form the country as far as a little hamlet on the river Sinope, 

 where the sandstone is seen to rest upon argillaceous slate. On the top of 

 the next hill towards St. Vaast, gravel beds, altogether about thirty feet in 

 thickness, rest upon porphyritic conglomerate and argillaceous slate ; and the 

 same thing occurs at two or three other places further on the road. 



In the department of Calvados this formation rests upon argillaceous slate 

 and grauwacke; except at Litry, where it rests upon coal measures. In the 

 department of La Manche, at Lestre, Montebourg, and in the neighbourhood 

 of Valognes, it rests upon quartz rock ; of which substance the gravel beds 

 so often mentioned are in fact principally, if not wholly, composed. This 

 may also be the case (though concealed by lias and the oolites) in the depart- 

 ment of Calvados, where the quartzose gravel is most abundant. That the 

 quartz rock occurs beneath the red sandstone formation, is seen at the several 

 denudations. 



Porphyritic Conglomerate. 



This rock constitutes the summit of the Bruyere deCrecy, a mountain near 

 Condc sur Noireau. It is composed of rounded quartz nodules, varying in 



M 2 



