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 A HISTORY OF BERKSHIRE seemed to bear the marks of roots, he adds, 'It is by no means improbable that the decomposition of vegetable matter and consequent formation of humus, and the various organic acids which arise from its gradual alteration into carbonic acid, may have had something to do with the concretionary action.' 1 A somewhat similar opinion has been expressed by the Rev. Dr. Irving. 2 Greywethers or sarsens are often to be seen at the corners of roads or used as stepping stones, and, as stated in the passage in Lyson's referred to above, they have been used as building stone. Mr. H. B. Woodward remarks that much of Windsor Castle is built of greywether sandstone. 8 In the sandy and clayey districts the older buildings often consist of a considerable variety of material. The tower of the church of Waltham St. Lawrence, for instance, is built mainly of chalk and of flints, but there are also several fragments of sarsen, a number of blocks of irony conglomerate and a few bricks. In addition to the works referred to above, several papers relating to Berkshire will be found in the recent volumes of the Quarterly 'Journal of the Geological Society f, the Proceedings of the Geologists' Association, and the Geological Magazine. 1 Pnc. Geol. Assoc. (1881-2), vii. 138. * Ibid. (1883-4), viii - 'S3- 3 Geology of England and Wales, ed. 2 (1887), p. 449. For further details of sarsens in Berkshire see T. R. Jones, Geol. Mag. (1901), viii. 54, 115. A bibliographic list of works treating of sarsens will be found at p. 124, and a paper by the same author in the Berks, etc. Arch. Journ. for July, 1901, vii. 54.