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Show 168 OLDER PLIOCENE PERIOD. [Ch. XII. preCi·p i·C es are 1a 1' d open on eac1 1 s1' de, varyinOo' from . 200 to 600 feet in height, and composed of inclined beds of shmgle, some. times separated by layers of sand, and m~re rar~ly by blue mi·c aceous mar1 . 'I'I1e pebbles in these stratified shmgles agree in composition with those now brought down from the Alps by the Vat· 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 ofthe river is dried up, the O'eologist has a good opportunity of examining a section of the ~trata, 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 consequence 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 torrents, 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 tl•is section in company with Mr. Murchison in 1828. Ch. XII.] TERTIARY STRATA AT NICE. 169 ties nearer to Nice. From these fossils it clearly appears that the formation belongs to the older Pliocene era. Such alternations of gravel and the usual thin layers of fine sediment may easily be explained, if \Ve reflect that the rivers now flowing from the Maritime Alps are nearly dried up in summer, and have only strength to drift along fine mud to the sea; whereas, in winter, or on the melting of the snow, they roll along large quantities of pebbles. The thicker masses of loam, such as that of St. Madeleine, may have been produced during a longer interval, when the river shifted for a time the direction of its principal channel of dischargeJ so that nothing but fine mud was for a series of years conveyed to that point in the bed of the sea opposite the delta. Uniform and continuous as the strata appear, on a general view, in the ravine of the Magnan, we discover, if we attempt to trace any one of them for some distance, that they thin out and arc wedge-shaped. We believe that they were thrown down originally upon a steep slanting bank or talusJ which advanced gradually from the base of Monte Calvo to the sea. The distance between these points is, as we have before mentioned, about nine miles, so that the accumulation of superimposed strata would be a great many miles in thickness, if they were placed horizontally upon one another. The strata nearest to Monte Calvo, which may be expressed by aJ are certainly older than those at bJ and the group b was formed before c. The aggregate thickness, in any one place, cannot be proved to amo~nt to 1000 feet, although it may, perhaps, be much greater. But it may never exceed three or four thousand feet; whereas, if we did not suppose that the beds were originally deposited in an inclined position, we should be forced to imagine that a sea, many miles in depth, had been filled up by horizontal strata of pebbles thrown down one upon another. At no great distance on this coast the Var is annually seen to sweep down into the sea a large quantity of gravel, which may be spread out by the waves and currents over a considerable space. The sea at the mouth of this river is now shallow ~ |