VIII.—Note on a Deposit of Allophane

Extract

In a denehole at Abbey Wood, near Plumstead, there is shown in the Chalk a fault of unknown extent (but probably having a throw of only a few inches), the plane of the fault roughly coinciding with one of the chamber walls. This wall has a strongly slickensided surface for some superficial yards, and extends to the roof of the chamber where the deposit of allophane is contained between the two fault faces. In some instances this mineral may be seen adhering to the fault face, and the roof of the denehole is composed of it for a length of about 7 feet by 2 feet wide; other patches are to be observed where the roof has been broken away, so that it seems evident the allophane extends horizontally some distance between the Chalk and the Thanet Sand; this latter is about 32 feet thick here. The deposit appears to be a roughly wedge-shaped mass, the greatest width being at the top of the Chalk, and from there diminishing downwards into tongues of the mineral extending between the two boundaries of the fault. Mr. J. L. Foucar, B.Sc., F.C.S., who kindly submitted a specimen to analysis for me, says: “I find the following to represent its composition:—