168 OLDER PLIOCENE PERIOD. [Ch. XII. 



precipices are laid open on each side, varying from 200 to 600 

 feet in height, and composed of inclined beds of shingle, some- 

 times separated by layers of sand, and more rarely by blue 

 micaceous marl. The pebbles in these stratified shingles agree 

 in composition with those now brought down from the Alps by 

 the Var and other rivers on this coast. 



The dip of the strata is remarkably uniform,, being always 

 southwards, or towards the Mediterranean, at an angle of 

 about 25. In summer, when the bed of the river is dried up, 

 the geologist has a good opportunity of examining a section of 

 the strata, as the channel crosses for many miles the line of 

 bearing of the beds, which may be traced to the base of Monte 

 Calvo, a distance of about nine miles in a straight line from 

 the Mediterranean *. It is usually impossible to determine 

 the exact age of such accumulations of sand and gravel, in con- 

 sequence of the total absence of organic remains. Their non- 

 existence may depend chiefly on the disturbed state of the 

 waters, where great beds of shingle are formed, which are 

 known to prevent testacea and fishes from living in Alpine tor- 

 rents, partly on the destruction of shells by the same friction 

 which rounded the pebbles, and partly on the permeability of 

 the matrix to water, which may carry away the elements of the 

 decomposing fossil body, and substitute no others in their place 

 which might retain a cast of their form. 



But it fortunately happens, in this instance, that in some 

 few seams of loamy marl, intervening between the pebble-beds, 

 and near the middle of the section, shells have been preserved in 

 a very perfect state of preservation, and these may furnish a 

 zoological date to the whole mass. The principal of these 

 interstratified masses of loam occurs near the church of St. 

 Madeleine (at c, diagram No. 29), where the active researches 

 of M. Risso have brought to light a great number of shells 

 which agree perfectly with the species found in much greater 

 abundance at a spot called La Trinita, and some other locali- 



* I examined tjiis section in cpmpany with Mr. Murchison in 1828. 



