TRANSACTIONS OF SECTION C. 505 
‘ Geological Magazine,’ in which the matter is treated at length. The green sands 
are simply dirty quartz-sand, with more or less of fine clayey material. By 
boiling in concentrated sulphuric acid the colour is quite destroyed, and the acid 
blackened: long boiling in caustic potash also removes the green colouring 
matter for the most part, but leaves some grains coated with a black amorphous 
substance, which the author considers to be humic acid, perhaps in combination 
with the silica. Both the black and the green materials, which appear only as 
incrustations of the grains under the microscope, often cementing smaller grains 
to the larger ones, can be removed by chemical reagents, and microscopic 
examination then shows a pure sand, made up partly of rounded, partly of 
angular grains, and exhibiting no trace of colour beyond a thin translucent pellicle 
of peroxide of iron after treatment with potash. Nota single grain of any green 
mineral has been found by the author in the green sands of either the valley-heads, 
or of the Middle and Lower Bagshot series, which both chemical and microscopic 
examination prove to owe the green, olive-green, and black coloration of their 
grains to amorphous matter of vegetable origin. Crenic and apocrenic acids are 
precipitated from the alkaline solutions in which these sands have been boiled, as 
well as from the waters of deep wells which are supplied from the Middle and 
Lower Bagshot strata where the green sands predominate; and the proportion of 
crenic acid increases with the increase of the depth of shade of the green colour of 
the sand as a whole. The paper on the action of humus acids by A. A. Julien 
(Am. Ass. Sci., 1879) is referred to, as confirming generally the author's 
conclusions on this part of the subject. The author has extended his observations 
to certain other sands, and shown that in them too the colouring matter is 
largely due to the influence of these vegetal acids. These include the grey sand- 
stones of the Molasse, near Lucerne, the green marls of the Upper Keuper, and 
a bed of dark green sand in the Woolwich and Reading beds near Croydon. 
Evidence is also given of the part they seem to have played in the ‘ mottling’ of 
certain sandstones, and the contemporaneous coloration of the red sandstones 
generally, by the formation of soluble ferrous salts, from which the peroxide of 
iron was afterwards precipitated by atmospheric oxidation in shallow waters. 
To their action is also attributed the bleaching out of iron from the surface sands, 
and its subsequent deposition at no great depth, to form the cementing material 
of the ‘ pan’ which is so frequently met with below the superficial layer of sandy 
districts in the Bagshot country, in Scotland, in America, and elsewhere. 
In the second part of the paper attention is drawn to some recent investigations 
by the author, of the origin of the siliceous cementing material of the sarsen-stones, 
which occur in the newest Bagshot strata and in the sandy strata of the Woolwich and 
Reading beds of the London basin. Reference is made to a fuller statement of 
the author's views in the paper before referred to. Looking to the facts (1) that 
(as the author has demonstrated) silica can be replaced and precipitated in the 
hydrated condition from soluble alkaline silicates by saturating their solutions 
with CO,; (2) that from such a gelatinous hydrate of silica, a glassy variety of 
silica separates out on removal of the water of hydration; (8) that microscopic 
examination of thin sections of sarsen-stones shows them to be composed of 
clear quartz-grains enclosed in a glassy siliceous matrix; (4) that felspars (as is 
well known) are decomposed by CO, with deposit of kaolin; (5) that kaolin is 
abundantly present in the siliceous matrix in which the quartz-grains are included 
—the author regards the induration of these sarsen-stones, and perhaps of siliceous 
sandstones and grits generally, as due to the accidental presence of felspar in the 
sand as it was originally deposited, and to the decomposition of this, with 
simultaneous liberation of silica and kaolin by carbon dioxide, and by the still 
stronger humus acids, which must have been copiously supplied by decomposing 
vegetation. 
7. On a Boulder from the Chloritic Marl of Ashwell, Herts. 
By H. George Forpuay, F.G.S. 
The boulders which are occasionally found in the Chloritic Marl in the workings 
for the so-called coprolites in Cambridgeshire and the neighbouring counties are 
