The Mineralogical Constitution of the Finer Material of the Bunter Pebble-Bed in the West of England

Abstract

I. Introduction. Much work has already been done on the pebbles and larger fragments of rock of the Budleigh-Salterton Pebble-Bed, but little or none on the finer material forming the matrix of this most interesting deposit. It occurred to me that a microscopic investigation of this finer material might bring out a few points of geological as well as mineralogical interest. Therefore it has been my endeavour to make as complete a mineralogical analysis of the sands as possible. Specimens were collected at intervals along the strike of the Pebble-Bed, from Budleigh Salterton on the southern coast of Devon to Fitzhead, north of Milverton, in Somerset; other sands, for comparison, were taken from the Red Rocks above and below. After treatment with acids, so as to remove the coating of iron-oxides from the grains, the sands were separated by means of heavy liquids into three parts:— ( a ) Heavy residue, with a specific gravity greater than 2·80. ( b ) The bulk of the quartz; and ( c ) The lightest part, with most of the alkali-felspar. II. Petrography of the Sands: Description and distribution of the Mineral Species. At the southern end of the outcrop of the Pebble-Bed, the matrix consists of incoherent material, each sand-grain being covered with a coating of red iron-oxide. On tracing the deposit northward, however, to the borders of Somerset, the bed loses much of its red colour, and in this area, owing to the presence of many masses