DEPOSITS OF HERTFORDSHIRE. 109 



led to their classification into Low-level or Valley-gravel and High- 

 level or Terrace-gravel. "Whether the High- and Low-level gravels 

 are of the same age, the result of excessive rainfall in what Mr. 

 Tylor calls the Pluvial Period ; * or whether the occasional insen- 

 sible passage from a glacial gravel into a river-gravel in the lower 

 part of a valley points to an older age for some river-gravels, is not 

 in all cases well made out. 



It remains now to give a brief account of the origin of the 

 subaerial deposits. That soils have resulted mainly from the dis- 

 integration of subjacent rocks is sufficiently well known. f This 

 disintegration is the result of the last denudation of the country, 

 the present wearing action of frost, rain, and rivers, together with 

 the deposits from organic agencies. Perhaps some of this disinte- 

 gration may have occurred at the time of emergence from the 

 glacial sea, as in the case of the strong loam, which often occurs 

 upon the Boulder-clay in a thin coating. Amongst the causes 

 which contribute to the formation of vegetable mould the common 

 earthworm has played an important part. Mr. C. Darwin has said;|: 

 that probably every particle of earth forming the bed from which 

 the turf in old pasture-land springs has passed through the intes- 

 tines of worms. Ants, likewise, are very efficient mould-makers 

 in dry soils, and no insignificant part must be played by other 

 animals, which bring to the surface new supplies of mineral matter 

 from below, and, mixing it with vegetable fibre, thus contribute 

 to form a fertile soil.§ 



III. Economic Importaistce. 



In considering briefly what has been the practical influence of 

 these Post-tertiary deposits, forming as they do in our county an 

 almost complete covering, sometimes of considerable depth, over 

 the older rocks, I shall confine myself to a discussion of that in- 

 fluence upon Land-valuation, Climate, and Public Health. 



Influence on Land Valuation, {a) Agricultural Influence. — A 

 comparison of a geological map of Hertfordshire with an agri- 

 cultural map will show how independent of the older substratal 

 rocks is the distribution of surface soils. The disti'ibution of 

 those substratal rocks will be shown on the geological map ; 

 while the agricultural map || will give a view of the general dis- 

 tribution of soils over the county. If a thii'd map showing 

 the Post-tertiary deposits were examined, it would be seen how 

 very close the agreement is between the distribution of the drift 

 beds and the agricultural soils. Had it not been for the covering 

 of drift, the main portion of our county, being on the Chalk f oi-ma- 



* Tylor, " Quaternary Gravels," ' Quart. Journ. Geol. Soc.,' vol. xxv, p. 57. 

 t Morton, ' Nature and Property of Soils.' 

 J 'Humble Creatures,' p. 23. 

 § 'Quart. Journ. Science,' April, 1876. 



II See Map in Young's ' Report to the Board of Agriculture,' and Map in 

 Dean's ' Improvement of Landed Estates.' 



