70 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



great number of small fragments. These are perfectly sharp, angular, 

 without any trace of having been worn, and follow in a continuous line 

 (but as it were stretched out) the curve of the pipe. Now it is evident 

 that this layer of friable sandstone must have been at one time con- 

 tinuous; that it has been fractured and its fragments disjointed and 

 spread out by some operation subsequent to its consolidation; and that 

 this must have been effected very gradually, and whilst in a supporting 

 medium, to admit of the perfect preservation of the sharp angles of 

 the fragments of so soft a stone, and to retain them in their original 

 relative position. Further, the large flints with which stratum "6" 

 always abounds are here, as usual elsewhere, scattered down (or rather 

 held up) on the nearly perpendicular sides of the pipe, and are not 

 accumulated at the bottom, as they necessarily would be if the sands 

 above had not existed to keep them in place by their pressure. With 

 few exceptions also these flints are the green-coated flints from the 

 Tertiary bed, and not the flints fresh out of the Chalk. When how- 

 ever the pipe traverses layers of flints, these large white unworn flints 

 are left after the chalk is dissolved, and remain in the outer clay- 

 seam usually forming the sides of the pipe : these flints are generally 

 in a vertical position at right angles to that which they held in the 

 chalk, — a downward pitch caused, I consider, by the slow downward 

 movement of the mass of matter in the pipe, whilst the flint was 

 gradually detached from its matrix, the one end being free and giving 

 way before the other. In a pipe on the Downs above Westerham, I 

 met with an instance where this pressure has been such that that 

 portion (about 10 inches) of the flint which projected into the angular 

 flint-rubble forming the sides of the pipe is snapped off from the 

 portion remaining attached to the chalk, and is slightly bent down- 

 wards, though it had not had time to become removed from the 

 other portion before the pipe-making action ceased. 



There are certain features common in all the sand-pipes under the 

 Thanet Sands ; a central core of sand always exists with an outer 

 layer of sandy clay with flints, which latter lie in all positions, per- 

 fectly independent of the relative weight of the materials, and more 

 often than otherwise with their longer axes directed downwards. The 

 sides of the pipes also are rubbed and striated vertically, presenting a 

 sort of shckenside, indicative of a slipping- downwards motion. The 

 lamination or bedding of the sands follows the sides of the pipe in 

 curved lines, gradually lessening in curvature as we ascend, until at 

 some feet above the chalk, the strata resume their horizontal position. 

 The sand above these pipes is also always looser and less compact 

 than usual. These characters are well exhibited in some large sand- 

 pipes at Grays Thurrock. In the following section (fig. 2) the 

 overlying indent of gravel further shows,-, that not only was the 

 action subsequent to the deposition of the sands, but that, in this 

 instance, it was subsequent to the spread of the gravel, c being a 

 small portion of a bed of gravel belonging to a higher level, and of which 

 this fragment, let down into the depression caused by the sand-pipe, 

 has escaped denudation. No later subsidence can have taken place, 

 as no further depression occurs on the surface of the gravel, and the 



