138 AMOUNT OP EARTH Chap. III. 



chalk, tlie insoluble matter, including a vast 

 number of unrolled flints of all sizes, has 



circular space, several feet in diameter, suddenly fel; in, leaving 

 on the field an open hole with perpendicular sides, some feet in 

 depth. This occurred in one of my own fields, whilst it was 

 being rolled, and the hinder quarters of the shaft horse fell in ; two 

 or three cart-loads of rubbish were required to fill up the hole. 

 The subsidence occurred where there was a broad depression, as 

 if the ssrface had fallen in at several former periods. I heard 

 of a hole which must have been suddenly formed at the bottom 

 of a small shallow pool, where sheep had been washed during 

 many years, and into which a man thus occupied fell to his great 

 terror. The rain-water over this whole district sinks perpen- 

 dicularly into the ground, but the chalk is more porous in certain 

 places than in others. Thus the drainage from the overlying 

 clay is directed to certain points, where a greater amount of cal- 

 careous matter is dissolved than elsewhere. Even narrow open 

 channels are sometimes formed in the solid chalk. As the chalk 

 is slowly dissolved over the whole country, but more in some 

 parts than in others, the undissolved residue — that is the over- 

 lying mass of red clay with flints, — likewise sinks slowly down, 

 and tends to fill up the pipes or cavities. But the upper part 

 of the red clay holds together, aided probably by the roots of 

 plants, for a longer time than the lower parts, and thus form,s 

 a roof, which sooner or later falls in, as in the above mentioned 

 five cases. The downward movement of the clay may be com- 

 pared with that of a glacier, but is incomparably slower ; and this 

 movement accounts for a singular fact, namely that the much 

 elongated flints which are embedded in the chalk in a nearly 

 horizontal position, are commonly found standing nearly or quite 

 upright in the red clay. This fact is so common that the work- 

 men assured me that this was their natural position. I roughly 

 measured one which stood vertically, and it was of the same 

 length and of the same relative thickness as one of my arms, 

 These elongated flints must get placed in their upright position, 



