34 
THE HALL. 
The more important quarries from which magnesian limestone- 
is obtained in this country are those of Anston. of Brodswortli. 
Cadeby, and Park Nook, near Doncaster, of Huddlestone near 
Sherburne, and of Smawse near Tadcaster, in Yorkshire ; while 
in Derbyshire the same stone of fine quality is obtained at 
Bolsover near Chesterfield, and in Nottinghamshire at Mansfield 
Woodhouse. 
In some of the chemical works on the Tyne the dolomites of 
the northern counties are used for the production of carbonate 
of magnesia ; while the magnesian limestones of Marsden haver 
been taken in considerable quantities to Sunderland for the 
preparation of Epsom salt (sulphate of magnesia). Magnesian 
limestone is now largely used in the manufacture of basic bricks 
and lining of converters for the Thomas-Gilchrist process (p. 100). 
Sandstones. — Cubes in Wall Cases IV., V., and VI. 
A sandstone consists essential^ of small grains of sand, mostly 
siliceous, cemented together into a solid rock. In some stones the 
grains cohere without any appreciable cement, but usually some 
kind of uniting medium is present. The nature of this cement- 
ing material is important, inasmuch as it determines to a large 
extent the durability of the stone ; those varieties being most 
durable in which the cement is siliceous, whilst those in which 
the agglutinating medium is calcareous or argillaceous, are 
generally lacking in durability. In many sandstones the grains 
of silica are accompanied by small fragments of other minerals,, 
such as felspar and mica, thus giving rise to varieties known as 
felspathic sandstone , micaceous sandstone, &c. 
The formation of sandstone generally is instructively illus- 
trated by a specimen of recently-consolidated sand from New- 
quay, in Cornwall, and many similar examples exist around our 
western coasts, where hills of blown sand prevail. The water 
percolating through the upper layers dissolves the carbonates of 
lime and of iron, which are re-deposited as cementing materials, 
on the evaporation of the water as it filters through the lower 
strata of the porous sand. 
In texture and colour sandstones are subject to considerable 
variation, according to the size of the grains and the nature of 
the cement. The red, brown, and yellow colours exhibited by 
many sandstones are due to the presence of peroxide of iron, 
either in an anhydrous or in a hydrated condition ; this material 
being usually present as a pellicle investing the sand-grains, or 
as a cement. Many sandstones and other rocks, though of 
brown or yellow colour on exposure to the oxidising influence of 
the atmosphere, are “ blue hearted,” or of a cold bluish tint 
when fresh-quarried : the blue tints may be due to the presence 
of ferrous carbonate, or of finely-divided iron-pyrites, or even of 
phosphate of iron. 
Although sandstones are highly valued as durable building 
materials, their use in London is far less extensive than that of 
