366 LECTURE XII. 



bedded in this mud. The pile-works thus belong' to an older 

 period than the upper peat^ and to a later period than the 

 lower peat with its mud covering. It is just this lower 

 peat bed which is connected with the lake formations of the 

 plains. 



"If, then^ the calculation of M. Troyon be correct, both the 

 formations which he compares should be of the same kind, 

 which is not the case. Nothing is more simple than the 

 formation of the sandy alluvia between Yverdun and the lake, 

 which are formed by the sand brought by the streamlets to the 

 lake, and which the billows cast upon the low banks ; nothing, 

 on the other hand, is more complicated than the plain between 

 Ohamblon and the lake. To the alluvia which first raised and 

 filled the bed of the lake, three successively formed dunes, and 

 two peat beds, have been added, which are separated from 

 each other by a layer of mud. This complicated stratification 

 required a much longer time, and the thirty-three centuries of 

 M. Troy on are quite inadequate for the chronology of the 

 pile works.'" 



I must here add, that the calculations of Troyon and Gil- 

 Heron are founded upon an erroneous basis. It is impossible 

 to calculate the time of the retreat from the horizontal distance ; 

 it is the vertical distance which is to be attended to. Let us 

 imagine a flat lake-basin a few kilometers in length, gradually 

 drying up. Around this lake are certain structures. The 

 surface of the water having sunk two feet, a space one kilo- 

 meter in diameter is dried up at one end. A structure is now 

 raised near the present water level. The lake sinks again two 

 feet, and in one thousand years the last structure is a kilo- 

 meter from the shore. But the lake-basin is narrow ; and of 

 the older structures, situated two feet higher, it would only be 

 the most distant which would furnish a correct result in cal- 

 culating its age — all others would yield a false date, as they 

 lie 800, 600, or perhaps only 100 meters in a horizontal dis- 

 tance from the recent structures. Gilheron would thus obtain 

 a different result were he to found his calculation upon the 

 nearer Neufchatel lake, and Troyon would, for a pile-work on 

 the southern bank of Chamblon, instead of the northern, have 



