Beet 

66 GHOGNOSY. 
siderite, gypsum, rock-salt, hematite, &c. This facility of replace- 
ment constitutes silica one of the most valuable petrifying agents in 
nature. Organic bodies which have been silicified retain often with 
the utmost perfection their minutest and most delicate structures. 
The student can usually detect quartz by its external characters, 
and especially by its vitreous lustre and hardness. When in the form 
of minute blebs or crystals, it may be recognised in many rocks with 
a good lens. Under the microscope it presents a characteristic — 
brilliant chromatic polarization, with no trace of any alteration of 
its borders; while caleedony displays a minute concentric radial 
structure giving a black cross between crossed Nicols. Where it is 
an original and essential constituent of a rock quartz it very com- 
monly contains minute rounded or irregular cavities or pores 
partially filled with liquid. So minute are these cavities that a. 
thousand millions of them may, when they are closely aggregated, 
lie within a cubic inch. The liquid is chiefly water, not uncommonly 
containing sodium chloride or other salt, sometimes liquid carbon 
dioxide and hydro-carbons.* 
Rock crystal and crystalline quartz resist atmospheric weathering 
with great persistence. Hence the quartz grains may usually be 
easily discovered in the weathered crust of a quartziferous igneous — 
rock. But corroded quartz crystals have been observed in exposed 
mountainous situations, with their edges rounded and eaten away.? 
The non-crystallized forms of silica are more easily affected. Flint 
and many forms of coloured calcedony weather with a white crust. 
But it is chiefly from the weathering of silicates (especially through 
— [Boox TI. 

é 
the action of organic acids) that the soluble silica of natural waters — 
is derived. Book III. Part Il. Section ii. § 7. 
Tridymite, in minute hexagonal tables (belonging according to 
von Lasaulx to the triclinic system), often somewhat rounded, and 
almost always grouped in twins, or still more in trins (hence the 
name), which are aggregated round and upon each other, has been 
met with chiefly among volcanic rocks (trachytes, andesites, &c.), 
both as an abundant constituent of those which have been poured 
out in the form of lava, and also in the ejected blocks of Vesuyius.? — 
Opal—the hydrated form of silica; amorphous, subtrans- 
lucent to opaque, containing 3 to 13 per cent. of water, with variable 
admixture of iron oxides, lime, magnesia, alumina, and alkalies, 4H. 
SD—6'D, Gr. 1:9—2:3. The opals have been formed from solution in 
water, or from the hydration of anhydrous silica. Noble opal, fire opal, 
common opal, and semi-opal are usually disseminated in veins and 
nests through rocks. Semi-opal occasionally replaces the original 
' See Brewster, Trans. Roy. Soc. Hdin. x. p.1. Sorby, Quart. Journ. Geol. Soe. 
xiv. p.493. Proc. Roy. Soe. Xv. p. 153; xvii. p. 299. Zirkel, Mikroskopische Beschaffenheit — 
der Mineralien und Cesteine, p. 89. Rosenbusch, Mikroskopische Physiographie, i. p. 80. 
Hartley, Journ. Chem. Soc. February, 1876. Auid c 
. The occurrence of fluid cayities in the 
erystals of rocks is more fully described in Part IT, § iv. of this Book. 
2 Roth. Chem. Geol. i. p. 94. 
* Vom Rath. Z. Deutsch, Geol. Ges. xxy. p. 236, 1873. 
