VI.—On the Occurrence of Websterite at Brighton

Abstract

On the 30th July last year I observed that a deposit of Websterite, subsulphate of alumina, had been cut into, in excavating for the new system of drainage in the Montpelier Road opposite the south end of Vernon Terrace. It occurs at a depthof 16 feet from the surface of the road, beneath a ferruginous deposit of varying depth, which overlies the chalk on the summit of the hill, consisting of ochreous clay with occasional flint-breccia and masses of hæmatite iron ore in some instances mammillated and associated with crystals of selenite. The iron ore is occasionally friable and of a cindery appearance, containing in its cavities angular pieces of chalk and occasional groups of crystals of selenite. The deposit of Websterite is about three feet wide at its junction with the overlying ferruginous mass, narrowing as it descends, apparently occupying a fissure in the chalk, which has at some time been filled with clay, or has been formed by some decomposing action on the chalk, the chalk intruding occasionally into the vein of Websterite. The mineral varies much in colour and appearance, consisting in some places of a soft white powder, which, I am informed by Sir W. C. Trevelyan, hehas observed in specimens at Newhaven, and which he has ascertained by the microscope to consist entirely of minute transparent crystals, the nature of which he believes has not yet been investigated; sometimes in masses of various size presenting the appearance of meerschaum, compact and structureless, or somewhat botryoidal in form, occasionally presenting a concentric structure, and rarely and only in a certain portion of the deposit exhibiting spherical concretions with a radiating structure. Specimens of these various forms I have presented to the British Museum. A mass of yellow clay with imbeddedchalk flints divides the summit of the vein of Websterite, and near the clay the mineral assumes the character of allophane, having a yellow ivory-like appearance, towards the chalk forming the wall of the vein of Websterite. The wall of the vein is marked by a dark line caused by the association of a soft black substance, oxide of manganese, with the Websterite.