Part Il. § vi] FRAGMENTAL ROCKS—PELITIC. 159 
purposes (Craigleith and other sandstones at Edinburgh, some of 
which contain 98 per cent. of silica). Glauconitic sandstone 
(green-sand)—a sandstone containing kernels and dusty grains of 
glauconite, which imparts a general greenish hue to the rock. The 
glauconite has probably been deposited through organic agency, as 
in the case of the green matter filling echinus-spines, foraminifera, 
shells and corals on the floor of the present ocean... Buhrstone—a 
highly siliceous, exceedingly compact though cellular rock (with 
Chara seeds, &c.), found alternating with unaltered Tertiary strata 
in the Paris basin, and forming from its hardness and roughness an 
excellent material for the grindstones of flour-mills may be mentioned 
here ; it probably has been formed by the precipitation of silica by the 
action of organisms. Arkose (granitic sandstone)—a rock composed 
of disintegrated granite, and found in geological formations of different 
ages, which have been derived from granitic rocks. Crystallized 
sandstone—an arenaceous rock in which a deposit of crystalline 
quartz has taken place upon the individual grains, each of which 
becomes the nucleus of a more or less perfect quartz crystal. 
Mr. Sorby has observed such crystallized sand in deposits of various 
ages, from the Oolites down to the Old Red Sandstone.? 
Greywacke.—A compact aggregate of rounded or subangular 
grains of quartz, felspar, slate, or other minerals or rocks cemented by 
a paste which is usually siliceous but may be argillaceous, felspathic, 
caleareous, or anthracitic (Fig. 13). Grey, as its name denotes, is 
the prevailing colour; but it passes into brown, brownish-purple, and 
sometimes, where anthracite predominates, into black. The rock is 
distinguished from ordinary sandstone by its darker hue, its hardness, 
the variety of its component grains, and above all by the compact 
cement in which the grains are imbedded. In many varieties so per- 
vaded is the rock by the siliceous paste that it possesses great tough- 
ness, and its grains seem to graduate into each other as well as into 
the surrounding matrix. Such rocks when fine-grained can hardly, at 
first sight or with the unaided eye, be distinguished from some compact 
igneous rocks, though a microscopic examination at once reveals their 
fragmental character. In other cases, where the greywacke has been 
formed mainly out of the débris of granite, quartz-porphyry, or other 
felspathic masses, the grains consist so largely of felspar, and the 
paste also is so felspathic, that the rock might be mistaken for some 
close-grained granular porphyry. Greywacke occurs extensively 
among the Paleozoic formations in beds alternating with shales and 
conglomerates. It represents the muddy sand of some of the Paleozoic 
sea-floors, retaining often its ripple-marks and sun-cracks. The 
metamorphism it has undergone has generally not been great, and for 
the most part is limited to induration, partly by pressure and partly 
by permeation of a siliceous cement. But where felspathic in- 
gredients prevail, the rock has offered facilities for alteration, and has 
1 See Sollas, Geol. Mag. iii. new ser. p. 539. 
2 Q. J. Geol. Soc. xxxvi. p. 63, See Daubrée, Ann. des Mines, 2nd ser. i, p. 206. 
