ROCK-FORMING MINERALS 5 
Varieties of Quartz.—The chief phanerocrystalline varieties are 
the following :—Aock-crystal, water-clear; Avanturine, rock-crystal, 
abundantly spangled with enclosed scales of mica or other mineral ; 
Amethystine Quartz, violet-coloured rock-crystal ; S7zoky Quartz includes 
dusty-brown to black (M/orzon) and paler brown to yellow (Cazrngorm) 
rock-crystal ; 4/z/ky Quartz is milk-white and nearly opaque, with a 
somewhat greasy lustre; Common Quartz, not transparent, white, but 
occasionally coloured, sometimes occurs with crystalline form, but is 
usually massive. The most important cryptocrystalline variety of 
quartz is Chalcedony. This is a secondary mineral, which may occur 
in almost any kind of siliceous rock. It frequently lines or fills vesicles, 
fissures, and other cavities in igneous rocks, and is common in metalli- 
ferous veins or lodes. It is translucent, and has a somewhat waxy 
lustre. The colour varies, the commoner kinds being white or grey, but 
brown or black, and yellowish-green and blue varieties are known. It 
frequently shows a banded structure, and often assumes nodular, 
mammillary, botryoidal, reniform, or stalactitic shapes, being obviously 
in such cases a deposition from aqueous solution. When a thin slice 
of a spherical concretion is seen under the microscope, chalcedony 
exhibits a finely fibrous radiating texture, and between crossed nicol 
prisms shows a black cross, which remains stationary while the slide 
is being rotated. Under chalcedony are included the following :— 
Carnelian, bright red, but sometimes yellowish ; Civrysoprase, apple-green ; 
Plasma, dark leek-green, but when spotted with carnelian known as 
fleliotrope or Bloodstone ; Agate, a variegated chalcedony, the colours 
being either banded or in clouds, or due to visible impurities. In 
Banded Agate the layers are wavy or zigzag, or concentric and more 
or less spherical, according to the conditions of deposition, and the 
shape of the cavity occupied by the mineral (Plate I. 1). In Clouded 
Agate the variously coloured portions are irregularly distributed. When 
visible impurities in a chalcedony assume moss-like or dendritic shapes, 
we have the variety known as Moss Agate. Onyx is an agate in which 
the coloured layers occur in even planes ; when one of these is dark 
brown, overlaid by a bluish-white layer, the mineral is used for cameos— 
the figure being carved in the white layer, while the dark layer serves 
fora background. Sardonyx is an onyx consisting of alternate layers 
of carnelian and opalescent chalcedony. /asfer is an impure chalcedony 
of various colours, red (due to ferric oxide) being the commonest. Flint 
is allied to chalcedony, consisting of cryptocrystalline silica, but rendered 
opaque owing to abundant impurities ; it has a marked conchoidal frac- 
ture. Cher¢ (including Hornstone) differs little from flint: the fracture 
is splintery rather than conchoidal. Flint and chert occur chiefly in 
calcareous rocks, in the form of nodules, layers, or irregular concretions. 
Weathering of Quartz and Chalcedony.—While the crystallised 
varieties of quartz remain practically unaffected by the chemical action 
of percolating water, the cryptocrystalline and amorphous forms of that 
mineral are not so resistant, but frequently “ weather” with a white crust. 
Opal is an amorphous mineral (z.e. devoid both of external crystalline 
