THE JURASSIC DEPOSITS WHICH UNDERLIE LONDON. 733 
very distinct horizons in the Chalk strata, and in this way to 
determine for the first time the limits and thicknesses of the 
_principal subdivisions of the Chalk below London. 
At a distance of 300 ft. below the top of the Chalk at Richmond, 
or 053 ft. from the surface, a stratum of very marked and peculiar 
character was met with. ‘The rock is of a creamy yellow tint (the 
chalk above and below it being pure white) and is seen to be made 
up of a number of nodular masses of a hard variety of chalk, each 
of these nodules being coated with a greenish deposit, while the 
interspaces between the masses are filled with a somewhat argillaceous 
matrix. In consequence of the presence of this argillaceous matrix 
the rock when immersed in water easily breaks up, the nodular 
masses of which it is composed separating the one from the other. 
The thickness of this stratum cannot exceed 5 ft. 
The nodular masses themselves contain 3°14 per cent. of matter 
which is not soluble in acids, while the portions of rock in which 
much of the matrix and the green coating of the nodules occurs were 
found to yield no less than 10-7 per cent. of insoluble matter. 
This rock agrees so precisely in all its characters with the stratum 
which has been so well described by Mr. Whitaker as the “ Chalk 
Rock,” that I cannot doubt of our haying in the Richmond well the 
representative of that interesting horizon. The Chalk Rock is not 
known immediately to the south of London, for I agree with 
Mr. Whitaker in thinking that the nodular bed recognized by 
Mr. Drew at several points in Surrey, and so well exposed in the 
“Rose and Crown” pit near Kenley, is at a considerably higher 
horizon. The Chalk Rock has been noticed, however, as a very thin 
layer in Devonshire, Dorsetshire, and the Isle of Wight, by Mr. 
Whitaker, and on the coast of Normandy by M. Hébert. 
About 20 ft. below the Chalk Rock, another bed of an indu- 
rated, but not nodular, character occurred. It is probably one of 
those harder bands sometimes found intercalated between the softer 
beds of chalk, like that for example which forms the roof of the 
underground workings at Chislehurst. 
At a distance of 137 ft.. below the Chalk Rock, however, a second 
horizon of great importance can be recognized. Instead of the 
ordinary chalk, we find a yellowish limestone of markedly crystal- 
line character. The material is so hard, indeed, that it often preserves 
perfectly slckensided surfaces, and it rings under the hammer. 
Some portions of the mass are nodular, with the nodules coated by 
a greenish deposit, as is the case with the Chalk Rock. Other 
portions of this mass are seen, in polished sections and in thin slices 
under the microscope, to be made up of more or less rounded frag- 
ments of an ordinary globigerina-chalk imbedded in a matrix 
composed of broken shells, Foraminifera, and Echinoderms; frag- 
ments of Belemnites can also be detected in this curious rock. 
The characters of this very remarkable rock are illustrated in 
Plate XXXIII. The whole stratum does not exceed 15 ft. in 
thickness. 
