PEBCOr.ATION, AND EVAPORATION. 51 



into the Lea basin, while none whicli falls in any pai't of the 

 Lea is likely to get into the basin of the Colno. 



Two causes combine to produce this eifect : one natural, the 

 other artihcial. The natural cause is the dip of the Chalk from 

 N.W. to S.E., from the Colne to the Lea. The artificial cause 

 is the loATcring of the plane of saturation of the Chalk in the Lea 

 valley by the large amount of water pumped from great depths. 



The Chalk is our great water-bearing stratum; firstly, owing 

 to its pervious nature, the whole of it being permeable, but its 

 permeability or water-bearing capacity decreasing towards its base ; 

 and secondly, because the water contained in it is held up by the 

 Gault Clay on which it rests, the Upper Greensand, which usually 

 in other districts separates these two formations, being absent or 

 reduced to a very thin bed. The Gault is usually spoken of as 

 an absolutely impervious bed of clay, but no rock (using the term 

 in its geological sense) which is in the least moist can be absolutely 

 impervious to moisture. The loss of water by percolation through 

 the Gault must, however, be very slight, and we may consider it 

 to be practically impervious. 



The permeable Chalk and the underlying impermeable Gault dip 

 slightly in Hertfordshire from N.W. to S.E., the gradient being at 

 least 60 feet to the mile in the N.W. and 30 feet to the mile in 

 the S.E. 



The inclination of our valleys, and consequently of our rivers, 

 and of the surface of the underground water in the Chalk is less 

 than this, varying from about 24 feet to the mile in the N.W. to 

 about 12 feet to the mile in the S.E. 



Some of the rain which percolates into the Chalk must filter 

 slowly through it, some must get into cracks or fissures which 

 are more or less vertical and are called "pipes," and some may be 

 rapidly carried away in " swallow -holes." Pipes are frequently 

 seen in vertical or oblique sections, as in chalk-pits and railway- 

 cuttings, when the chalk is not protected from the percolation of 

 water into it by impermeable beds of clay reposing upon it, but 

 when there are such impermeable beds no "pipes" are seen, 

 showing that water does not then percolate into and dissolve it 

 to any appreciable extent. These "pipes" may in course of time 

 be so enlarged, and the chalk in which they are formed may be 

 so much dissolved away, that they become "swallow-holes," the 

 difference between "pipes" and "swallow-holes" being that in 

 pipes the insoluble residue of the chalk is left more or less in its 

 original position, the beds above gradually sinking and taking the 

 place of the soluble residue, so that no open channel is left, and in 

 "swallow-holes" the chalk is washed away with all its contents, 

 such as flints, and an open channel is formed, down which large 

 quantities of water may freely pass. 



At intervals in the Upper Chalk, layers of flints occur, with 

 usually a few feet of chalk between them. They are absent below 

 the Chalk Kock which forms the boundary between the Upper and 

 the Middle Chalk. 



