TYLOR AMIENS GTIAVEL. 121 



section, page 257, Phil. Trans. 18G4, the height of the rails at Montiers 

 is marked 130 feet ; it should be 99 feet, according to M. Guillom. 

 It is to be regretted that Mr. Prestwich was supplied with incor- 

 rect figures of the relative levels of the ground about Amiens, as 

 the introduction of such errors in the section must have materially 

 affected Mr. Prestwich's theoretical views, as he says, " The upper 

 section at Montiers, which I discovered in 1861, was conclusive 

 as to the relative ages of the gravel" (p. 248, Phil. Trans. 1864). 



In the plan, Plate v., Phil. Trans. 1864, accompanying Mr. Prest- 

 wich's memoir, the bare chalk is shown as invariably separating the 

 upper and lower gravels all the way from Amiens to Abbeville ; but 

 I have never seen a case of the kind. 



It must be remembered that so much gravel has been removed 

 during the last four years, that the sections are now much clearer ; 

 and, with the assistance of the accurate measurements of M. Guillom, 

 present examiners have a great advantagokover their predecessors, in 

 examining the structure of the gravel near Amiens. 



I cannot suppose that Mr. Prestwich would now separate the 

 Montiers gravels, seen in and above the railway- cutting at Montiers, 

 from those in the Great Montiers pit, and into two horizons, as there 

 is only a difference of twenty-two feet between the height of the 

 gravel on the top of the railway-cutting and that in the Imperial 

 road. As nearly the whole space between these two points has 

 now been excavated, the continuity of the gravel is now proved. 



When Mr. Prestwich supposed that there was a continuous bare 

 band of chalk separating the gravel in the railway-cutting at Mon- 

 tiers from the gravel near the Imperial road, and that the top of the 

 railway- cutting was (according to the measurement in his section, 

 page 257, Phil. Trans. 1864) sixty feet above the Imperial Eoad, he 

 very naturally took a diflPerent view of the relations of the gravels 

 from what we must take at the present time, with the additional 

 information we possess. 



The section on Plate lY., therefore, appears to destroy Mr. Prest- 

 wich's argument, on which he has constructed a division of the 

 gravels at St. Acheul and at Montiers into upper vaUey-gravels and 

 lower valley-gravels, of different ages, and situated on different hori- 

 zons, separated, as he supposed, by a band of bare chalk from each 

 other, — the upper valley-gravel being supposed to have been deposited 

 before the excavation of the last fifty feet of the Somme valley, which 

 excavation, he considered, preceded the deposition of the gravels near 

 the Imperial road, Montiers. 



The character of the surface of the chalk at Montiers has been 

 discussed at full length in this paper, and shown to be concave at 

 the pits ; while it is represented as highly convex at Montiers by 

 Mr. Prestwich and Sir C. Lyell. 



In the long section CD (Plate IV. fig. 3), the St. -Acheul gravel, at 

 a height of 140 feet above the sea, is shown to be separated from 

 the loess at Longueau, at a height of ninety feet, by an escarpment 

 of bare chalk. The tramway (Plate IV. fig. 1), passing from the Im- 

 perial road to the railway, crosses one of the supposed bands of 



