84 STRUCTURAL AND FIELD GEOLOGY 
Some varieties are very fine-grained or compact, so that the constituents 
are not visible even with the help of a lens. Rocks of this kind (such as 
jint and chert) usually occur in the form of nodules, irregular aggregates, 
veins, or layers, especially in limestones. They are compact and homo- 
geneous, have usually a dull, horny-like aspect, cannot be scratched with 
the hardest knife, and do not effervesce with acid. Lydian-stone is 
another close-grained siliceous rock, which usually contains carbonaceous 
matter, so that it is dull grey or even black. It 1s commonly associated 
with greywacké, clay-slate, or schistose rocks, and is often traversed by 
numerous ramifying veiniets of white quartz. These characters and its 
great hardness suffice to distinguish it. All the compact siliceous rocks 
referred to are differentiated from certain compact igneous rocks ( /fedszfes) 
with which they might possibly be confounded, by their infusibility 
before the blowpipe. 
Granular siliceous rocks are typically represented by sandstones and 
conglomerates—the determination of which presents no difficulty even to 
a beginner. The hard, round, subangular or angular grains of a sand- 
stone consist chiefly of quartz. The student should be able, with the 
help of his pocket-lens, to detect the fragmental character of even the 
finest-grained sandstone. The bedded character of the rock and the 
general aspect of the strata with which it is associated should leave him 
in no doubt as to its nature. Sandstones, of course, differ greatly as 
regards their hardness and durability—some are much more closely 
compacted than others. The nature of the cementing material, as we 
have already learned, also varies. It is just possible that a fine-grained 
oblitic limestone might be mistaken for a sandstone, but the relative 
softness of the former, and the readiness with which it effervesces with 
acid, at once betray its character. It 1s unnecessary to add a word as to 
the determination of conglomerates—everyone is familiar with the 
appearance of such consolidated gravels. The only other eminently 
siliceous rock that calls for notice is guartzite. This is simply a much 
indurated sandstone—the grains of the rock being cemented by silica. It 
is, therefore, exceedingly hard, and breaks with a splintery or conchoidal 
fracture, and usually shows a somewhat glassy lustre. ‘The student may 
note further that in the case of quartzite and many sandstones which 
have silica for their binding material, the component grains are so firmly 
cemented together that they do not separate but break across, so that the 
face of the fracture is smooth’and often glistening or glassy ; whereas, in 
ordinary sandstones, a fresh fracture is dull and has a rough feel, owing 
to the rock separating between the grains and not through them. 
The Felspathic Rocks offer a wide range as regards hardness, texture, 
and structure. Some are soft and more or less earthy ; others are hard, 
distinctly crystalline, fine-grained, smoothly compact, or glassy ; while yet 
others are fragmental. Again, the crystalline varieties may be schistose, 
or their ingredients may be confusedly aggregated. Many of the harder 
felspathic rocks cannot be scratched with a knife, but usually their hard- 
ness is less than that of the siliceous rocks. In the fresh state none of 
them effervesces with acids. 

