176 Mr. De la Beche on the Geology of the 



from north to south, from between the villaj^es of Levens and La Roquette 

 near the mouth of the Vesubia to the sea. 



They consist of an irregular mixture of sand, sandstone, and conglomerate 

 composed of rounded pebbles of granite, gneiss, slates of dilferent kinds, red 

 sandstone, porphyry, compact, grey litnestones, various sandstones, and in fact 

 of every rock which the Alps can furnish. 



The cement is sand or sandstone, composed of more comminuted portions of 

 the same rocks as the pebbles, with much mica. 



The picturesque appearance of the deep valleys that intersect these beds is 

 well described by Mr. Allan*. "Near the banks of the Var," he observes, 

 "\ have seen clitfs cut in the gravel of at least 400 feet high, and quite per- 

 pendicular. The operations of the various little streams which occupy the 

 water-courses in wet weather, — for in dry weather there is little or no water 

 to be seen in them, — are very remarkable : sometimes I have followed them 

 up, having barely room to squeeze myself through between the perpendicular 

 walls, and found the cut suddenly terminate in a circular aperture, like a deep 

 well, into which a little stream precipitated itself. In the Vallon Obscur, the 

 opening is in some places not more than three or four feet, and the walls rise 

 to at least one hundred feet on each side, fringed at top with shrubs and trees." 



The sand and sandstones alternate with the conglomerates ; they all dip to 

 the south, except on the flanks of a mountain, as at Mont Cao. They are 

 undisturbed, though the rocks on which they rest are much contorted. 



2. Blue marl {sub-appenine marl-mama turchina of Brocchi). 



Though this marl is generally blue or of a lead colour, yet it is lig'ht brown 

 near the upper portions ; it is used as brick earth, and abounds in shells, many 

 of which agree with the recent. See the lists of M. Rissof and Mr. Allan;};. 



M. Risso attributes to this bed the relative position which is here assigned to 

 it, and cites Dr. Buckland, perhaps erroneously, as considering it the equivalent 

 of the plastic clay of Paris, which M. Brongniart found at Marseilles : the marl 

 of that place, however, contains abundance of lignite and fresh-water produc- 

 tions ; that of Nice contains no fresh-water shells, and but little lignite. The 

 section § will afford an idea of its appearance in the valleys of La Madelaine and 

 Magnan : it occurs also in the valley de la Mantega, and above the close part 

 of the Vallee Obscur ; its most common and characteristic fossil is a Natica||. 



* See Trans, of Royal Soc. Edin. vol. viii. p. 442. 

 f Hist. Nat. de I'Europe Merid. vol. i. p. 126. 



X Trans. Royal Soc. Edin. vol. viii. p. 455. § Plate XXIII. fig. 2. 



II This grey marl or clay is observed to alternate, and to be intimately connected with the con. 

 glomerate at Vintimiglia. 



