TYLOR QUATERNAEY GRATELS. 



97 



tell from the appearance of the surface that such a cavity had existed. 

 There must have been a wash of gravel all over the sand to have 

 furnished the material to fill up such cavities as that shown in 

 fig. 31 ; and since that wash of gravel there is no indication of any 

 subsequent deepening of the pipes in the chalk. 



The Thanet sands (6), 25 feet thick at B, at the north of fig. 31, 

 are seen lying in situ on the chalk. 



Where the formation of the great pipe A has undermined the 

 Thanet sands, h, the latter have fallen down to the bottom of the 

 pipe A, which is funnel-shaped, and filled up 20 feet without any 

 mixture of gravel whatever. The sands moved from their proper 

 position are indicated by the letters b' b' ; the bottom part of the 

 pipe A is not shown in this drawing. Under C, masses of sand, b b, 

 are shown falling into the pit by gravitation, from the removal of the 

 chalk by chemical action. The gravel series c c is 30 feet thick 

 above A, and is similar in mineral character to the covering bed e. 



Mr. Prestwich has explained the formation of pipes by chemical 

 agency in the chalk. 



Fig. 32 is a drawing of other beak-Hke projections in the chalk 



Fig. 32. — Section near Knighton Mill, Isle of Wight. 



where it has been acted upon by denudation previously to the depo- 

 sition of the gravel near Knighton Mill, Isle of Wight. 



Fig. 33 (p. 98) is a section of a fissure or pipe in the highly in- 

 clined chalk on Brading Down, Isle of Wight, which is a remarkable 

 contrast to the ordinary pipes which are filled with sand or gravel. 



The general conclusions to be drawn from the sections of the 

 Thames-valley fossiliferous deposits appears to be that they are a 

 series of variable sands, loams, and clays intercalated between the 

 upper and lower gravels. 



The false-bedded sands are below the loams and clays at Erith, 

 and above them at Grays, while at Crayford they are in the middle 

 of the series. This may simply arise from the principal current 

 changing from one side of the old river to the other during the de- 

 position of the Thames Quaternary series. By this means, where the 

 current became more rapid, sands would be deposited, and where 



VOL. XXV. PART I. . H 



