298 Prof. T. G. Bonney—On the Ightham Stone. 
south-eastern side of a little combe we again find the hard white sand- 
stone, capping a spur of the hill, and about three yards below it the 
softer bed crops out. I have examined microscopically the hard bed, 
which recalled to my mind, before I had read the passage quoted 
above, some of the hardest ‘greywethers.’ It consists almost wholly 
of fairly angular to subangular quartz grains, commonly about 
0125” in diameter, which are probably derived from some granitoid 
rock, but do not exhibit any minor peculiarities worthy of remark. 
These are cemented by secondary quartz, in no great quantity, which 
is sometimes, but not always, in perfect optical continuity with 
the original grains, which often appear enclosed by coloured rings 
when viewed with crossed Nicols; probably from causes similar to 
that which produces them in separate grains and chips. There are 
a few grains showing a minute chalcedonic structure and one or two 
of brown or nearly black iron oxide. Parts of the slide also have 
a rather dirty look—in short, the rock is very like one of the hard 
grits or less perfect quartzites that one finds among the older 
Paleozoic rocks. 
The other rock mentioned above is peculiar. It is of a glaucous 
green colour, varying somewhat in depth, and it weathers on the 
outside to a rusty brown. This suggests that the tint is due toa 
silicate of iron, and in the process of weathering the cementing 
mineral appears to be partially removed ; for the discoloured portion, 
often an inch or so in depth, is less solid than the rest of the rock. 
Occasionally a smooth face of a fragment—probably an old divisional 
surface—has a kind of glazing of the green-coloured substance. 
The stone probably occurs én situ ata slightly higher level in the 
Folkestone Sands (here about 100 feet thick) than the white rock, for 
shallow pits have been dug in search of it over the upper part of 
Oldbury Hill! Boulders also were formerly abundant over the 
surface in many places, resulting from past denudation. Some also, 
like erratics, as Mr. Harrison informed me, have been transported 
northward and eastward for more than a mile, and may be found 
lying on the lowland of Gault almost as far as the base of the 
escarpment of the North Downs.’ 
The mode of working for the masses of green rock suggests that, 
like the Sarsen-stones of the Tertiary and the Cornstones of older 
rocks, they are of concretionary origin. The rock is not generally 
seen in situ, but Mr. Hale took me to a sandpit where two masses 
were exposed. The opening was some 20 feet deep. In the lower 
part the sand was markedly false-bedded, being in the upper fairly 
horizontal, with alternating coarser and finer bands of white, pale 
red, or brownish colour. Toa depth of perhaps a couple of yards . 
the usual flagey masses of sand cemented by limonite were common. 
The two blocks of green ‘quartzite’ were nearly on the same level, 
1 Mr. Harrison informs me that when the rock was extensively worked for the 
Metropolitan roads, some half-century since, it always occurred in detached masses, 
is that the opener of a pit might get sometimes nothing, sometimes a rich return, for 
S pains. 
2 The surface of the ground at the Camp is from about 500 to 600 feet, of the 
Gault about 300 feet above the sea. 
