76 STRUCTURAL AND FIELD GEOLOGY 
matrix (Plate XXI. 3). Quwuartzite is composed of grains of 
quartz cemented by silica to form a very hard finely granular 
or compact rock. It occasionally shows traces of false- 
bedding and diagonal lamination, and is obviously an altered 
sandstone. The fracture is usually splintery, but in very 
compact varieties tends to be conchoidal. The rock may be 
white, grey, yellowish, or reddish, and even occasionally bluish 
or greenish. It occurs in thin and massive beds intercalated 
among crystalline schists.* Quartz-schist is a quartzite, in 
which a foliated structure has been developed—the planes of 
foliation being glazed over with scales of white mica. 
Argillaceous Rocks.—The best known members of this 
group are the clay-slates. C/ay-s/ate is a finely granular or 
compact clay-rock. It divides into thin plates which some- 
times coincide with the original planes of deposition, but 
usually cross these at various angles. The colour of the rock 
may be blue, green, grey, purple, brown, or red. Slate is com- 
posed mainly of argillaceous matter (silicates and hydrous 
silicates of alumina), but many other ingredients may be 
present, such as quartz, mica, felspar, chloritic and carbon- 
aceous matter, rutile, iron-oxides, pyrite, etc. Some of these 
minerals are original constituents of the clay, others have 
been developed in the rock subsequent to its formation as a 
sediment. The scales of mica and crystals of rutile, for 
example, have obviously been developed along the superin- 
duced planes of cleavage. In some slates well-marked con- 
spicuous crystals (chiastolite, andalusite), are disseminated 
irregularly, and are clearly of secondary origin—the minute 
fragmental particles of the slate having been pushed aside 
during the gradual growth of the crystals, 
There are many varieties of clay-slate, such as Roofing-slate, with 
smooth cleavage ; Pemczl-slate, a soft slate of pure composition used for 
writing on slate ; Whet-slate or Novacultte, a highly siliceous slate, with 
indistinct cleavage, used for sharpening knives; A*nthractte-slate and 
Alum-slate, carbonaceous slates which often contain marcasite and 
pyrite—the decomposition of which gives rise to the formation of alum ; 
Knotted- or Sfotted-slate (see Plate XXII. 1), a slate containing little 

* Now and again, however, sandstone and unconsolidated sand of 
relatively recent age have been partially converted into quartzite by the 
infiltration of silica in solution, 

