52 J. HOPKINSON — HERTFORDSHIRE RAINFALL, 



The interstices in the layers of flints being more open than in 

 the solid chalk between them, and the dip of the Chalk being at 

 a higher angle than the dip of the plane of saturation, water 

 finding its way into these layers will flow more readily, or more 

 quickly, along them than it will flow in the solid chalk, which 

 offers a very great frictional resistance to its passage. The dip 

 of the Chalk being from N.W. to S.E., there must be a natural 

 tendency for water to flow in that direction. 



The water-parting between the catchment-basins of the Colne 

 and Lea from near Dunstable on the N.W. to a little beyond 

 Hatfield on the S.E., trends nearly in the same direction as the 

 Chalk dips, but at this point it takes a turn to the S. From 

 a N. and S. direction it turns a little to the W. near Northaw, 

 and continues in the same direction to near Barnet. If, therefore, 

 water is flowing in the Chalk from N.W. to S.E., it must, to the 

 S. of Hatfield, flow, underground of course, out of the basin of the 

 Colne aud into that of the Lea. 



On the opposite or W. side of the Colne basin, as far S. as 

 Harefield at least, there is no such cause for an accession of water 

 underground, so that the underground reservoir of the basin of the 

 Lea will obtain an accession of water at the expense of that of 

 the Colne, while the loss to the Colne will not be compensated for 

 from an outside source. 



The Lea basin, on the other hand, probably loses more water on 

 the E. than it gains from the Colne on the W., for the boundary of 

 its watershed on that side runs in a N.E. to S.W. direction. This 

 is almost on the strike of the Chalk, or at a right angle to its dip, 

 and therefore the natural flow of the water in the Chalk will be 

 almost directly away from the basin of the Lea on its south- 

 eastern boundary. 



The abstraction of a large quantity of water from the Chalk 

 in the Lea basin must tend, by lowering the plane of saturation, 

 to enlarge the subterranean basin, causing the water - parting 

 underground to gradually recede beyond the water-parting above 

 ground. How far this is the case in this or any district, can only 

 be ascertained by actual hydro-geological surveys such as have been 

 carried on by Mr. Joseph Lucas. 



That the plane of saturation in the Chalk sho\ild not be unduly 

 lowered is most important for the welfare of Hertfordshire, our 

 chief industries — agriculture, watercress-growing, brewing, and 

 paper-making — being dependent, for their successful carrying on, 

 upon the presence of a certain amount of moisture in the subsoil 

 in the case of agriculture, and upon an abundant supply of water 

 in the Chalk in the case of the other industries. 



Our rivers are chiefly fed by the natural overflow of the water 

 in the Chalk, which finds its way to the surface in springs, most 

 of which are in the banks and beds of the rivers, their flow being 

 thus constantly augmented without accession of water from any 

 tributary streams. 



Owing to the permeable character of the Chalk, the water in any 



