Vol. 50.] BORINGS AT CTTLFORD, WINKFIELD, WARE, AND CHESHUNT. 505 



613. Lighter-coloured, glauconitic and micaceous sandstone, of much 

 less specific gravity. This Mr. Hill recognizes as an Upper 

 Greensand Sponge-bed. " The chief constituents are quartz- 

 sand, sponge-spicules and glauconite-grains, with a few shell- 

 fragments and foraminifera. These materials are closely 

 packed together, and are cemented partly by crystalline calcite 

 and partly by silica in the amorphous colloid condition. Both 

 calcite and silica seem to permeate the whole rock, and some- 

 times masses of calcite are surrounded and isolated by the 

 silica. Both seem to have been derived from the organic 

 remains of which the rock must in the first instance have been 

 largely composed, and to a certain extent the two minerals 

 have replaced each other. Thus the siliceous spicules are now 

 almost entirely replaced by calcite-casts (? of their canals) ; only 

 a few remain in their original state, and in these the spicular 

 canal is usually filled with glauconite. Colloid silica in a 

 globular form is abundant. The glauconite-grains, though 

 abundant, are rather small." 



621. A light-coloured siliceous stone of low specific gravity. On 

 examining a slide of this Mr. Hill found it to be an interesting 

 rock. He describes the matrix as " almost entirely colloid 

 silica, partly in minute masses like tiny bits of gum arabic, 

 partly in discs or globules, the latter irregularly scattered in 

 patches. In this matrix are dispersed small grains of quartz, 

 broken sponge-spicules, with a few shell-fragments, and a few 

 foraminifera. The siliceous matrix being comparatively clear, 

 the other ingredients stand out conspicuously." 



622, 628. Calcareo-siliceous rocks, heavier than the last, and 

 externally resembling calcareous malmstone. Describing a 

 slide cut from 622, Mr. Hill says " the mass of it consists of 

 definite, separate calcite-crystals of irregular size, set in a matrix 

 of fine calcareous material. There are many spicules, the silica 

 of which is replaced by calcite, and there are also many 

 residuary spicular canals in glauconite. Quartz-grains, though 

 abundant, are well separated, but no colloid silica of any kind 

 was observed. Glauconite-grains are small and not abundant." 

 628 is a similar rock. 



632. A fine, dark-grey, silty clay. This, as would be expected from 

 its external aspect, is regarded by Mr. Hill as a Gault clay. 

 Under the microscope the mass of it is seen to be a fine, 

 brownish, inorganic material, with much fine quartz-sand. 

 Small glauconite-grains occur, but not abundantly, and there 

 are also broken glauconitic spicular casts. Thread-like fila- 

 ments (? mica-flakes in transverse section) are common, and 

 there are some larger crystals whose striation and frayed ends 

 suggest felspar. 



658. A dark- grey clay, with a soapy feel ; effervesces with hydro- 

 chloric acid, and is therefore somewhat calcareous. 



670 to 725. Nine samples of dark-grey clays; all very fine-grained, 

 homogeneous Gault clays. 



790. A fine, compact, dark-grey clay. This was sliced and examined 



