1865.] 



FOSTER AND TOPLEY MEDWAT GRAVELS. 



455 



a full account of this subject to Mr. Prestwicli's admirable paper 

 " On the Origin of the Sand and Gravel Pipes in the Chalk of the 

 London Tertiary District " *. 



Fig. 5. — Section of " Pipes" in Iguanodon Quarry (^May 1864). 



a. Kentish Eag. b. Sandgate Beds c. Gravel. 



In the case of the pipe shown in fig. 5, few persons would hesi- 

 tate to admit that the following was the mode of its formation : — 

 Gravel was deposited on the surface of the Sandgate Beds, then rest- 

 ing horizontally on the Kentish Eag ; afterwards part of the Kentish 

 Rag was dissolved away by the percolation of water charged with 

 carbonic acid, and the Sandgate Beds and gravel sank down to fill 

 the vacant space. In many of the quarries we find every gradation 

 between small pipes and large ones ; and if it is admitted that the 

 small pipes were formed in the manner described above, the same 

 mode of formation must be allowed for large ones. 



It seems at first not a little remarkable that where beds have 

 been let down in pipes into limestone, they are generally separated 

 from the Kmestone by a bed of clay. The only exception of which 

 we are aware is in the case of some pipes in the Magnesian Lime- 

 stone, described in the ' Geologist' for 1860, by Mr. Kirkby, who 

 says (p. 297), " The pipes are found in the limestone beneath the 

 sand beds. I have never noticed them where the sand is absent ; 

 and though they are sometimes filled with clay, or a mixture of 

 clay and sand, yet in these instances a thin layer of sand is always 

 the immediate cover of the limestone ; nevertheless, the quarrymen 

 assert that pipes have occurred in other parts of the hill where the 

 limestone is immediately covered by the Boulder Clay." 



The clayey covering of the Chalk (" claj'-with-flints "), and also 

 the clay lining the Chalk pipes, may be the direct result of chemical 

 action upon the chalk ; but facts about to be described render it at 

 least probable that a horizontal covering of clay, whether formed by 

 chemical action or actual deposition, may help the formation of 

 pipes. 



* Quart. Jouvti. (tcoI. i>oc. rol. xi. IS')."), p. Gi. 



