The Occurrence of Colloid Silica in the Lower Chalk of Berkshire and Wiltshire

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The existence in the Lower Chalk of beds containing a certain proportion of pure disseminated silica is not a new discovery, the occurrence of such material having been proved by Messrs. Way and Paine in 1851; but we are not aware that it has ever before been found in such quantity, or that any detailed investigation of its mode of occurrence has previously been made. The beds which are the subject of the present communication were discovered during the examination of the Lower Chalk of Berkshire and Wiltshire in 1887–88, for the purposes of the Geological Survey, and we are indebted to the Director-General of the Survey for permission to make use of the stratigraphical information then obtained. Beds of siliceous chalk were first observed near Wantage and Letcombe Bassett, and attracted attention from their hard and compact character; they occurred near the horizon at which the Totternhoe Stone is found further north, and it was at first thought possible that one of them might be the representative of that stone. Specimens were therefore submitted to microscopical examination, and were found to be different from Totternhoe Stone and from any other samples of Lower Chalk previously examined by us. The exact stratigraphical position of these beds was subsequently ascertained by means of the excellent section exposed by the cutting on the Didcot, Newbury, and Winchester line, near Chilton.