Brick-making employs a considerable number of hands in various parts of the county; the glacial and other superficial deposits on the chalk area frequently yielding excellent brick-earth; while the London clay may be employed for brick-making anywhere in the south-eastern districts. Bricks from the London clay, which is naturally blue, turn yellow or white after burning in consequence of the combustion of the organic colouring matter; but many of those from the glacial clays are of a full rich red At Pepperstock, near Caddington, however, there are manufactured certain very hard, heather-coloured bricks, which are much favoured for house-building in north-western Hertfordshire, although their colour compares very unfavourably in the matter of effect when contrasted with the “brick-red” of the more ordinary kinds. As we approach the Gault plain of Bedfordshire numerous cement works may be seen at the edge of the chalk-marl near the northern borders of the county, some of which may be within the county itself. Chalk is much worked, as at Hitchin and elsewhere, for lime; and, as already mentioned, is dug largely by the farmers for “chalking” their fields. It is to such diggings that the deep circular pits (now generally ploughed over) to be seen in many arable fields are due. The Totternhoe stone has been, and perhaps still is, quarried locally for building in some parts of the northern districts of the county. 13. Minerals--An Exhausted Industry. Having referred in the last section to brick-making, lime, cement, and Totternhoe stone, very little remains for mention in the present one; as the absence of mines is one of the features of Hertfordshire and the adjacent counties. Reference may, however, again be made to the so-called coprolite beds of the chalk-marl which were worked in the neighbourhood of Hitchin in the early part of the first half of last century as a source of phosphoric acid for agricultural manure. The irregularly shaped black nodules of phosphate of lime occur crowded together in a comparatively thin bed. They were dug out, and washed from the marl in which they were embedded on the spot in large circular tanks through which a wheel was made to revolve by horse-labour; and then carted away to be converted by a chemical process into superphosphate. After the excavation of the coprolite bed from one strip of a field, the marl from the next was thrown in, and the top soil replaced, so that the land was left in as good, or even better, condition than previously. The industry was continued till all the beds situated at or above a level which it paid to work were exhausted.