 ParrIl.§ vi] CRYSTALLINE ROCKS—STRATIFIED. 117 
Siliceous Sinter (Geyserite, Kieselsinter), the siliceous deposit © 
made by hot springs, including varieties that are crumbling and 
earthy, compact and flinty, finely laminated and shaly, sometimes 
dull and opaque, sometimes translucent, with pearly or waxy lustre. 
The deposit may occur as an incrustation round the orifices of 
eruption, rising into dome shaped or even columnar elevations, or 
investing leaves and stems of plants, shells, insects, &c., or hanging 
in pendent stalactites from cavernous spaces which are from time 
to time reached by the hot water. When purest, it is of snowy 
_ whiteness, but is often tinted yellow or flesh colour. It consists of 
silica 84 to 91 per cent., with small proportions of alumina, ferric 
oxide, lime, magnesia, and alkali, and from 5 to 8 per cent. of 
water. 
Flint (Silex, Feuerstein).__A grey or black excessively compact 
rock with the hardness of quartz and a perfect conchoidal fracture, 
its splinters being translucent on the edges. Consists of an 
intimate mixture of crystalline insoluble silica and of amorphous 
silica soluble in caustic potass. Its dark colour, which can be 
destroyed by heat, arises chiefly from the presence of carbonaceous 
matter. Flint occurs principally as nodules, dispersed in layers 
through the upper chalk of England and the north-west of Europe. 
It frequently encloses organisms such as sponges, echini and bra- 
chiopods, and has been deposited from sea-water, at first through 
organic agency, and subsequently by direct chemical precipitation 
round the already deposited silica. (Book IJ. Part IT. Section iii.) 
Chert is a name applied to impure varieties of flint, other brittle 
varieties are known as hornstone, which, under the microscope, how- 
ever, presents a crystalline structure. 
Some of the other varieties of silica occurring in large masses may 
be classed asrocks. Such are jasper, common quartz, and ferruginous 
quartz. ‘These occur as veins traversing both stratified and unstrati- 
fied rocks; also as beds associated with the crystalline schists. 
With them may be grouped Lydian-stone, a black or dark coloured, 
excessively compact, hard, infusible rock, with splintery fracture, 
occurring in thin, sharply defined bands, split by cross joints into 
polygonal fragments, which are sometimes cemented by fine layers of 
quartz. It consists of a mixture of silica with alumina, carbonaceous 
materials, and oxide of iron. It occurs in bands in the Silurian and 
later paleeozoic formations interstratified with ordinary sandy and 
argillaceous strata. As these rocks have not been altered the 
bands of Lydian-stone may be of original formation, though the 
extent to which they are often veined with quartz shows that they 
al in many cases been permeated by siliceous water since their 
eposit. 
Quartzite isa granular and compact aggregate of quartz, which 
has been produced by the metamorphism of sandstone. It will be 
described in connection with the schistose rocks among which it so 
frequently occurs. 
