ALLUVIAL COVERING. 45 



being visible only when the tide is out. Let it be remembered, how- 

 ever, that the bottoms of meres, or fresh water lakes, are often below 

 the level of the sea; and this might be the case with many of our 

 meres. Besides, might there not be a subsidence of the bogs them- 

 selves, where they open towards the sea, or any large river, without 

 any general sinking of the land ? In piercing through the alluvial 

 beds near Hull, it is found, that beneath the peat bog there is a thick 

 bed of sand and clay, mixed with water, forming a kind of quicksand ; 

 and in sinking at Sutton and Mablethorpe, near the boggy islets 

 above mentioned, there is found beneath the boggy stratum about 

 twenty feet of very soft clay or mud. Now it is easy to see, that 

 wherever the bog is exposed to the tide, or to the river, with any 

 such soft bed under it, especially if that bed be of a considerable 

 thickness, there must be a partial subsidence of the bog; for the soft 

 clay, or mud, or sand, having nothing to support it, will burst out, 

 or be protruded, from under the bog ; and the tide or current washing 

 away the part first protruded, will make room for more ; while the 

 weight of the boggy stratum itself, and of the alluvium incumbent 

 over it, must serve to press out the soft matter from beneath it; and 

 thus a subsidence of the bog and its covering will take place, which 

 will be greater or less, according to the softness and thickness of the 

 stratum below, and the weight of the alluvium above. Some of the 

 bogs connected with the Humber are known to have actually subsided 

 a great number of feet, particularly those between Thorne and Gowle; 

 and the boggy banks of the Humber may all have experienced a sim- 

 ilar depression. Where the exposed bog itself is remarkably soft, 

 the protrusion or compression of its own substance will sink the whole 

 mass so much the lower. 



We have dwelt longer on this part of our subject than may seem 

 necessary; but the importance of elucidating topics that have been 

 involved in so much obscurity, may serve as an apology for the length 



M 



