158 - GEOGNOSY: . ——«*([Boox IL | 
Sandstone (Grés)—A rock composed of consolidated sand. As — 
in ordinary modern sand, the integral grains of sandstone are chiefly — 
quartz, which must here be regarded as the residue left after all the 
more decomposable minerals of the original rocks have been carried 
away in solution or in suspension as fine mud. The colours of sand- 
stones arise, not so much from that of the quartz, which is commonly 
white or grey, as from the film or crust which often coats the grains 
and holds them together as a cement. Iron, the great colouring 
ingredient of rocks, gives rise to red, brown, yellow, and green hues, 
according to its degree of oxidation and hydration. 
Like conglomerates, sandstones differ in the nature of their 
component grains, and in that of the cementing matrix. Though 
consisting for the most part of siliceous grains, they include others 
of clay, felspar, mica, or other mineral ; and these may increase in 
number so as to give a special character to the rock. Thus sand- 
stones may be argillaceous, felspathic, micaceous, calcareous, &e. 
By an increase in the argillaceous constituents, a sandstone may 
pass into one of the clay-rocks, just as modern sand on the sea-floor — 
shades imperceptibly into mud. On the other hand, by an augmen- 
tation in the size of the grains, a sandstone may become a grit, or a 
pebbly or conglomeratic sandstone, and pass into a fine conglomerate. 
A piece of fine-grained sandstone seen under the microscope looks 
like a coarse conglomerate, so that the difference between the two 
rocks is little more than one of relative size of particles. 
The cementing material of sandstones may be ferruginous, as 
in most ordinary red and yellow sandstones, where the anhydrous or 
hydrous iron oxide is mixed with clay or other impurity—in red 
sandstones the grains are held together by a hematitic, in yellow 
sandstones by a limonitic cement; argillaceous, where the grains are 
united by a base of clay, recognizable by the earthy smell when 
breathed upon; calcareous, where carbonate of lime occurs either as 
an amorphous paste or as a crystalline cement between the grains; 
siliceous, where the component particles are bound together by a 
flinty substance, as in the exposed blocks of eocene sandstone known 
as “orey-weathers” in Wiltshire, and which occur also over the 
North of France towards the Ardennes. 
Among the varieties of sandstone the following may here be 
mentioned. I'lagstone—a thin bedded sandstone, capable of being 
split along the lines of stratification into thin beds or flags; Mica- 
ceous sandstone (mica-psammite)—a rock so full of mica-flakes 
that it splits readily into thin lamina, each of which has a lustrous. 
surface from the quantity of silvery mica. This rock is called 
“fakes” in Scotland. Freestone—a sandstone (the term being 
applied sometimes also to limestone) which can be cut into blocks 
in any direction, without a marked tendency to split in any one 
plane more than in another, Though this rock occurs in beds, each 
bed is not divided into lamine, and it is the absence of this minor 
stratification which makes the stone so useful for architectural - 


