SANDSTONES. 13 
Kellaways Hock, Lower Calcareous Grit and Portland Sands. 
These appear in all cases to be cemented by calcareous matter, 
and hence the Doggers when exposed are liable to rapid decay. 
That the masses were hardened in situ is shown by the occurrence 
of laminae of bedding and false-bedding in some of the spheroidal 
doggers ; and when split up the upper and lower portions often 
present the aspect of " pot lide."* They furnish as it were one 
stage in the formation of sandstone, evidence of T7hich is well 
shown in some of the quarries in the Lower Calcareous Grit of 
Berkshire, and in the sands associated with the Collyweston Slate. 
Colour of Rocks. 
The limestones and clays, which at the surface present various 
shades of buff and brown, are usually bluish -grey at a depth the 
difference being due generally to the peroxidation of the protosalt 
of iron. (See p. 8.) 
The blue or grey colour of strata, especially of clays, may be 
due to the pre&ence of carbonaceous matter ; it is also due in some 
cases to carbonate of protoxide of iron ;t and in others to sulphide 
of iron. J Six or seven per cent of iron-ore is commonly found 
in the Jurassic clays. Analyses of Kimeridge Clay show in 
some cases carbonate of iron and bisulphide of iron in quantities 
of not more than 1 to 2 per cent. 
Silicate of iron gives a green colour to the cores of some iron- 
stones ; and in the form of Glauconite it gives a similar colour to 
many sands and sometimes to calcareous rocks, of which examples 
occur in the Portland Beds. 
Of pure white earthy limestones like some varieties of Chalk, 
we have examples in the Purbeck Beds, Portland Beds, Kimeridge 
Clay (occasional bands), and Great Oolite ; in the same beds the 
layers are sometimes compact like the White Lias, which occurs 
in the upper part of the Rhsetic Beds. 
The Inferior Oolite is blue-hearted in places in Dorsetshire, but 
not as a rule markedly so when quarried in the west of England. 
Nor do we find that the Great Oolite in the country near Bath 
presents these bluish tinges ; in fact where quarried at a depth 
near Corsham it comes out as a buff or yellow freestone. Under 
London, however, the rock is bluish-grey, and contains 2 * 40 per 
cent, of sulphide of iron in the form of pyrites.^ (See p. 7.) 
In the Midland counties we find more distinct evidence of 
bluish-limestones even when the beds are quarried near the 
surface. In this region there are protecting clays in the Upper 
Estuarine Series, in the Great Oolite Clay, and in the Boulder 
* See also De la Beche, Researches in Theoretical Geology, p. 95. 
f G. Maw, Quart. Journ. Geol. Soc., vol. xxiv. pp. 356, 357, 366 ; Judd, Geol. 
Rutland, p. 176, and Quart. Journ. Geol. Soc., vol. xl. p. 741. 
% See also paper by Ebelmen (quoted by Judd), Bull. Soc. Geol. France, ser. 2, 
tome ix. p. 221 ; and A. H. Church, Quart. Journ. Chem. Soc., ser. 2, vol. ii., p. 379. 
C. Homersham, Quart. Journ. Geol. Soc., vol. xl. p. 726. 
