260 FOSSIL PLANTS OF [Ch. XV. 



Nearly the whole of this Lower Molasse is freshwater, yet some of 

 the lowest beds contain a mixture of marine and fluviatile shells, the 

 Cerithium, margaritaceum, a well-known Lower Miocene fossil, being 

 one of the marine species. Notwithstanding, therefore, that some of 

 these Lower Miocene strata reach an elevation of 6000 or even 7000 

 feet above the sea, the deposition of the whole series must have be- 

 gun at or below that level. For ages, in spite of a gradual sinking of 

 the coast and adjacent sea-bottom, the rivers continued to cover the 

 sinking area with their deltas ; but finally, the subsidence being in 

 excess, the sea of the Middle Molasse gained upon the land, and ma- 

 rine beds were thrown down over the dense mass of freshwater and 

 brackish-water deposit, called the Lower Molasse, which had pre- 

 viously accumulated. 



The great change of level above alluded to must be borne in mind 

 if we would account for a phenomenon by which geologists have been 

 much puzzled ; namely, the fact that in the " nagelflue," as the con- 

 glomerates are called by the Swiss, pebbles of gneiss, granite, and 

 porphyry are common, and yet no such rocks now enter into the 

 structure of the Alps. Along the original coast-line, when the peb- 

 ble-beds of the Lower Miocene were forming, there may have been 

 hills of granite and gneiss more than a thousand feet high, but when 

 the subsidence had continued for a long series of years, these would 

 all be gradually submerged and covered over by fluviatile sediment ; 

 for the effect of a general depression going on at a faster rate than 

 the accumulation of sediment is to cause the shore-line to retreat 

 inland, the sea occupying successively old zones of coast. In the 

 present period we see at the southern base of the Alps in Italy, hills 

 of gneiss and porphyry of moderate height, although rocks of this 

 class form at present no part of the chain itself, and these crystalline 

 formations might be submerged and buried under deltas derived from 

 the detritus of the higher Alps, if the level of the whole region were 

 to be lowered by another great downward movement. 



As I have already stated, the inferior portion of the Swiss Lower 

 Miocene, called Aquitanian by Heer, may best be studied on the 

 northern borders of the Lake of Geneva between Lausanne and Ve- 

 vay, where the contiguous villages of Monod and Bivaz are situated. 

 The strata there, which I have myself examined, consist of alterna- 

 tions of conglomerate sandstone and finely laminated marls with fossil 

 plants. A small stream falls in a succession of cascades over the 

 harder beds of puddingstone, which resist, while the sandstone and 

 plant-bearing shales and marls give way. From the latter no less 

 than 193 species of plants have been obtained by the exertions of 

 MM. Heer and Gaudin, and they are considered to afford a true type 

 of the vegetation of the inferior subdivision of the Lower Miocene 

 formations of Switzerland — a vegetation departing farther in its 

 character from that now flourishing in Europe than any of the higher 

 members of the series before alluded to, and yet displaying so much 



