1853.] DE LA CONDAMINE — DRIFT IN HUNTINGDONSHIRE. 273 



containing bones at the base (according to the workmen) . Hollows 

 in the surface of this bed are filled with marl, in which are the fol- 

 lowing land and freshwater shells, with fragments of vegetable sub- 

 stances, and an indeterminable fragment of bone : — 



*Pupa margiaata. Bithinia tentaculata. 

 *Helix hispida (var. concinna). Planorbis marginata. 

 *Valvata piscinalis. spirorbis. 



Succinea oblonga. *Cyclas cornea. 



Limneus pereger. *Pisidium amnicum. 



* These occur also in the upper bed of sand. 



Nearly five-sixths of the specimens which I procured were Pupce, — 

 a remarkable numerical preponderance which appeared likely to throw 

 light on the precise circumstances of the deposit. I therefore gathered 

 up indiscriminately the shells left on the bank of the river after a 

 slight flood, and found that that was the exact ratio of terrestrial to 

 aquatic shells ; and on examining the recent alluvium, I found land 

 shells entirely wanting at the base, rare in the middle, but exceed- 

 ingly abundant at the top. The inference seems to be, that their 

 presence in large numbers indicates a level a little above the ordinary 

 height of the water ; and this might well be anticipated when we con- 

 sider that a land-flood will sweep off from the meadows the dry and 

 empty shells, while by drowning others it prepares a fresh supply for 

 its successor. I conclude then that the seam and patches of marl 

 indicate such a level, and were accumulated in pools bordering on a 

 river which discharged itself through the present valley. This view 

 is favoured by the direction of the diagonal lamination f of the sand 

 both above and below the marl : see section 2. In the upper bed of 

 sand are a few shells of the species marked with an * in the preceding 

 list. The " stratula " of coarse sand pass upwards indistinctly into 

 the unstratified bed of fine gravel ; and on the latter rests 2 feet of 

 sandy soil with a few flints dispersed in it. 



I interpret the phenomena of this section thus : after the deposit 

 of the rough gravel, a river met the sea probably not much below 

 this point, and deposited the lower bed of sand, till either this filling 

 up, or a slight movement of elevation, produced a temporary sub- 

 aerial condition. The marl was then formed from the decomposition 

 of shells, and washed into its present position out of pools higher up 

 the valley, as would appear (1) from its amorphous condition while 

 containing perfect shells ; (2) from its impure state and mixture with 

 loam ; and (3) from the absence of calcareous rocks in most of the 

 country through which the river would run. A movement of sub- 

 sidence then commenced, during which the river again threw down 

 sand and shells. And here it must be observed that the fall of the 

 present river between Huntingdon and St. Ives is 3 feet per mile, 

 which would give a sufficient velocity for the transport of sand, 

 although, from artificial interference wdth its course, the Ouse de- 

 posits only loam. The subsidence continued until this spot reached 



f May not the word stratula be coined to describe those smaller subdivisions 

 of strata which are frequently oblique ? I should then say that the stratula here 

 dip to the east. 



