_Geology._--The rocks of Hertfordshire belong to the shallow syncline known as the London basin, the beds dipping in a south-easterly direction. The two most important formations are the Chalk, which forms the high ground in the north and west; and the Eocene Reading beds and London Clay which occupy the remaining southern part of the county. On the northern boundary, at the foot of the chalk hills, a small strip of Gault Clay and the Upper Greensand above it falls just within the county. The lowest subdivision of the chalk is the Chalk Marl, which with the Totternhoe Stone above it, lies at the base of the Chalk escarpment, by Ashwell, Pirton and Miswell to Tring. Above these beds, the Lower Chalk, without flints, rises up sharply to form the downs which are the easterly continuation of the Chiltern Hills. Next comes the Chalk Rock, which being a hard bed, lies near the hilltops by Boxmoor, Apsley End and near Baldock. The Upper Chalk slopes southward towards the Eocene boundary previously mentioned. The Reading beds consist of mottled and yellow clays and sands, the latter are frequently hardened into masses made up of pebbles in a siliceous cement, known locally as Hertfordshire puddingstone. The London Clay, a stiff blue clay which weathers brown, rests nearly everywhere upon the Reading beds. Outliers of Eocene rocks rest on the chalk at Micklefield Green, Sarrat, Bedmont, &c. The Chalk is often covered by the Clay-with-flints, a detrital deposit, formed of the remnants of Tertiary rocks and Chalk. Glacial gravels, clays and loams cover a great deal of the whole area, and the Upper Chalk itself has been disturbed at Reed and Barley by the same agency. Chalk was formerly used for building purposes; it is now burned for lime. Reading beds and London clay are dug for brick-making at Watford, Hertford and Hatfield. Phosphatic nodules have been excavated from the base of the Chalk Marl at several places along the outcrop; the Marl is worked for cement.