180 



The Geology of the Berks l^ Hants Extension, 



unworn, and a few tertiary pebbles. The Brick-earth is the same 

 sandy-clay which forms the matrix for the flints below. It fills up all 

 inequalities, its bedding following the curves of the upper surface 

 of the Clay- with-flints, even when it decends into a pothole like 

 that in the figure. The Brick-earth is free from flints, but contains 

 Sarsen stones in large masses ; these sometimes lie on the top 

 surface of the Clay- with-flints, but are never imbedded in it. 



The irregularities of surface in the Chalk are not longitudinal 

 gullies, but potholes. This was clearly seen while the cutting was 

 in progress. The depth of some of the pot holes from the surface 

 of the ground exceeds 30 feet, and they sometimes extend for as 

 much as half this depth in the Chalk. 



The theory of Lyell, that potholes in the Chalk are the result of 

 the chemical action of water charged with carbonic acid derived 

 from decaying vegetable matter, receives considerable support here. 

 In the Chalk there is a tabular layer of flint forming a well marked 

 level line. Wherever this line approaches the side of a pothole, it 

 bends downwards, while apparently still in the solid chalk. This 

 is just what would happen if the chalk were dissolved away by per- 

 colating water. The Chalk below the layer of flint would be pro- 

 tected, except near the sides of the potholes, when the wasting of 



the Chalk would let down 



the flint layer. Fragments 

 of the tabular flint were 

 sometimes seen containing 

 the downward line in the 

 pothole, and in one instance, 

 fig. 8, the fragments extend- 



* ito"- ■* ed quite across the pothole, 



Fig- 8. lying in a festoon in the clay 



within a few inches of the bottom. Had the deepening of the 

 pothole gone on, the connection of the middle part of the curve 

 would soon have been broken, and the tabular fragments, except 

 those near the sides would have been lost in the mass of larger 

 flints. 



Everything seems to indicate a quiet subsidence of the overlying 



