ROCKS 67 
Flint is a hard grey or black rock, composed of amorphous 
or chalcedonic quartz—the dark colour being due to carbon- 
aceous matter. It breaks with a marked conchoidal fracture, 
and is translucent along the sharp cutting edges. Its most 
characteristic occurrence is in the form of nodules, layers, and 
vertical ramifying or vein-like masses in white chalk. 
Its precise mode of formation is not quite clear, but it would appear to 
be partly of organic, partly of chemical, origin. Sponges and other 
organisms secrete soluble silica from sea-water, and when they die 
additional silica is deposited upon and within their skeletons and 
exuvize. Calcareous shells, and even the chalk itself in which these are 
embedded, have often been partially or wholly replaced by silica, so that 
silica in a soluble form must have been diffused to some extent through 
the calcareous ooze of Cretaceous seas. Probably the silica was largely 
derived from the skeletal remains of sponges, which flourished in great 
abundance during the formation of the Chalk. Chert is a somewhat 
impure kind of flint, of not uncommon occurrence in limestones belonging 
to the older geological systems (Palzozoic), and, like it, probably partly 
of organic, partly of chemical, origin. In some cases, however, it possibly 
represents the deposits of thermal springs. ornstone is a somewhat 
similar rock; it is more brittle than flint. Lyditan-sfone is a mixture of 
silica and clay, usually with carbonaceous or ferruginous matter. It is 
black, purplish, red, or dark blue, very hard and compact, and often much 
cracked and rent, the small fissures being usually filled with white 
quartz. It occurs in thin beds and layers in the older Palzeozoic systems, 
and in some cases, at least, contains remains of radiolarians (Radiolarian 
Chert), so that such rocks would appear to represent the radiolarian ooze 
of ancient seas. 
Ironstones are sometimes of chemical, sometimes of 
organic, origin, or partly both. Occasionally they occur in 
the form of beds or layers interstratified with other derivative 
rocks, or of nodules and nedular masses embedded chiefly in 
argillaceous deposits. They are frequently met with also 
occupying fissures and irregular cavities. Lzmonzte, when 
approximately pure, is a compact fibrous or stalactitic 
aggregate, in which form it usually occurs in veins and 
cavities. When appearing as a bedded rock, it is usually 
earthy and porous, and crowded with impurities. It forms 
the hardpan which so frequently appears under marshy 
ground (Bog Jron-ore), and often occurs as a- lacustrine 
formation in layers of small spherical bodies (Oodzézc or Pea 
Lron-ore). Hematite, already described as a mineral, appears 
