224 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jail. 4. 



On the otter side of the hill, in proceeding from Hatch Green to 

 Dinstead and Fish Pond Farm, there are several other swallow holes. 

 Altogether the district is a very interesting one in this respect, and, 

 although I have seen larger streams absorbed elsewhere, I know of 

 no other locality where so many may be seen within so small an area, 

 or where the phsenomenon is more striking*. 



It would appear that two conditions are essential for the formation 

 of swallow holes : the one, that there should be streams formed at 

 such a level that they have to pass over a surface of country higher 

 than that of the main valleys of drainage ; and the other, that the line 

 of water level in the mass of calcareous strata in which the swallow 

 holes are formed should be below the level at which the streams drill- 

 ing the swallow holes are absorbed. The action is therefore dependent 

 upon the tendency which the water has to seek the lowest water- 

 level. Consequently wherever a predisposing cause f, such as a crack 

 or fissure, may exist, the water will tend to escape through any avail- 

 able subterranean channels to the lower water-level and then to the 

 river-channels, in lieu of flowing over the surface to the same ultimate 

 point, as it would do in the case of passing over strata not acted 

 upon by water holding, as usual with all surface waters, carbonic 

 acid in solution. 



* Amongst other places in the chalk around London where swallow holes may 

 be observed, I would instance in particular the neighbourhood of Farnham, and 

 the districts immediately north of Newbury, S.W. of Hungerford, and N. of 

 Barnet. 



t lu this case the old Tertiary sand-pipes may have determined the formation 

 of some of these existing swallow-holes. 



