DEPOSITS OF HERTFORDSHIRE. Ill 



and ash ahoiind. Still further south the elm is common. The elm 

 and oak liourish much better where the Chalk is covered by Ter- 

 tiaries, but if drift gravel is present we find only larch and fir.* 



{b) Mineral Value. — The economic products obtained from the 

 Post-tertiary beds are not numerous. Some of the sands can be 

 used for mortar, metal-moulding, glass-making, and tempering 

 pottery and brick-clays ; while the gravels are useful for foot- 

 paths, roadways, filter-beds, and concretes. The clays and brick- 

 earths are manufactured into bricks, tiles, and drain-pipes ; the 

 drift flints are calcined for pottery admixtures ; and the chalky 

 Boulder- clay was once extensively used for marling fields. 



Influence on Climate. — We have seen that it is to the drift that 

 "we owe the well- wooded appearance of our county, and thus the 

 drift exerts an indirect influence upon our climate. Trees exert 

 a most important local influence on climate, acting, according to 

 Becquerel,f as frigorific causes in three ways — 



( 1 ) By sheltering the ground against solar irradiation. 



(2) By the cutaneous transpiration of their leaves. 



(3) By the multiplication, by their branches, of radiating surfaces. 



Trees also affect the distribution of rainfall, by causing precipi- 

 tation of moisture when cooling saturated currents of air passing 

 over them. By their roots, also, they have a powerful action on 

 springs, facilitating percolation, and thus draining the surface soil, 

 and removing moisture beyond the reach of evaporation. This 

 insures the permanence and regularity of natural springs in or 

 near woods. | But the drift has also a more direct influence upon 

 atmospheric humidity. The Mid-glacial sands and gravels form 

 a natural drainage for superflcial soils and thus lessen evaporation 

 and consequent refrigeration, and contribute to the dryness and 

 warmth of the air.§ 



Sanitary Influence. — Fuller said of Hertfordshire : " It is the 

 Garden of England for delight ; men commonly say that such as 

 buy a house in Hertfordshire pay two years' purchase for the aire 

 thereof." This salubrity, says Mr. Clutterbuck, is due to the 

 geological condition of the greater part of the county, gravel upon 

 chalk. II That such is the truth is further proved by the labours of 

 Mr. Whitaker and Dr. Buchanan, who found a very marked con- 

 nexion between wetness of soil and the consumption death-rate, 

 and noticed the great importance of the coverings of permeable 

 di'ift-gravels upon such impervious soils as the London Clay.^ 



Nor must we forget here the importance of the drift in con- 

 nexion with water-supply. Although as water-bearing strata the 



* Clutterbuck, "Agriculture of Hertfordshire," ' Joum. Roy. Agric. Soc.,' 

 vol. XXV, p. 314. 



t ' Des climats et de I'influence qu'exercent les sols boises et non-boises.' 



X Boussingault, " Economic Rurale," ' Zeitschrift des Oest. Ingenier uiid 

 Architekten Vereins,' 1875, pp 157-165. 



§ 'Quart. Journ. Science,' Jan. 1871. 



II 'Journ. Roy. Agric Soc.,' vol. xxv, p. 303. 



IT ' Geol. Mag.,' Nov. 1869. 



