
4 STRUCTURAL AND FIELD GEOLOGY 
of a large number of sedimentary rocks. ‘These, as we shall learn, are 
simply residual products—that is to say, they have been derived from 
the disintegration and degradation of pre-existing rock-masses; and 
quartz, in consequence of its superior durability, its great abundance, 
and wide distribution, naturally forms a dominant ingredient of con- 
glomerates, greywackés, sandstones, etc. 
In coarsely crystalline rocks, quartz, even when it shows 
no external crystalline form, is quite readily recognised by 
its other physical characters—namely, by its hardness, its 
uneven or conchoidal fracture, its vitreous lustre, and the 
absence of any trace of decomposition. In coarse-grained 
granite, for example, it appears like a kind of transparent 
cement, filling up the straggling spaces between the other 
mineral ingredients, which it thus seems to bind together. 
In certain other eruptive rocks, as in quartz-porphyry, 
pitchstone, etc., it often occurs as conspicuous, corroded, 
but occasionally well-formed crystals, disseminated through 
a groundmass of fine-grained materials (see Plate IV. 4). 
The best-developed quartz-crystals met with in eruptive rocks, 
however, are found in certain curious irregular cavities which 
frequently appear in granite. The walls of such cavities are 
usually lined with fine crystals of the several mineral con- 
stituents of the rock, amongst which hexagonal prisms and 
pyramids of quartz are commonly prominent (see Plate XI. 2). 
In finely crystalline rocks, the presence of quartz can only be 
determined by microscopic examination. In thin slices it appears 
limpid, water-clear, and quite unaltered. It shows no trace of cleavage, 
but is traversed by numerous irregular cracks. The surface appears 
smooth, and neither bounding edges nor internal cracks are pronounced. 
When crystals of the mineral are present (as in quartz-porphyry), they 
usually show lozenge-shaped outlines with rounded angles. According 
to the thickness of the slice, and the direction of the section, the polari- 
sation colours vary in intensity, being grey, white, yellow, orange, 
blue, or green. 
Enclosures of other minerals are common in quartz. [In large crystals 
these are often visible to the naked eye (see Plate J. 2).] Under the 
microscope even the smallest granules of the quartz of eruptive rocks, 
such as granite, may appear crowded with inclusions of rutile, apatite, 
and other minerals. The quartz of granite also usually contains 
numerous minute fluid cavities, more or less irregularly disseminated 
through the mineral, while the quartz of pitchstones, rhyolites, and 
quartz-porphyries frequently encloses minute quantities of glass or stone 
<see Plate IV. 4). 
