Sandstone, a rock formed of grains of sand, often intermixed with coarse pebbles, cemented together by the infiltration of calcareous, argillaceous, ferruginous, or silicious substances. This, with long continued pressure, has converted the collections of sand into solid rock. Sandstone strata occur through all the geological formations from the metamorphic group upward, and the hard quartz rocks of this group are now understood to be altered sandstones. Those formations of the stratified rocks in which layers of sandstone prevail are often specially designated by this name. Beds of very coarse pebbles are known as pudding-stones and conglomerates. (See Conglomerate.) The Potsdam sandstone, near the base of the Silurian rocks, is extremely hard, close-grained, and quartzose, often occurring in broad sheets and little intermixed with other strata. Its beds in several places in New England, New York, and E. Pennsylvania attain a thickness of more than 300 ft.; and at Potsdam in St. Lawrence co., N. Y., a thickness of 70 ft. is exposed in the quarries. The rock is remarkable for its uniform thickness in broad sheets; masses are taken out 30 ft. square and 2 ft. or more thick, perfectly solid and smooth.
Divisional planes are exposed by the hammer and wedges, and the thickness of the sheets may be reduced to an inch. The general color is yellowish brown, variously shaded in the different layers. (See Potsdam.) Many other sandstones are extensively employed for building, some of which are easily quarried in sheets, of agreeable color, and well suited by their hardness and sharpness of grit for architectural ornaments. Such especially are the grits or harder sandstones of the coal measures, usually brownish yellow or whitish. The formations known as the old red and new red sandstones afford quarries of superior building stones; but they also contain many layers of very inferior stone. The old portion of the capitol at Washington is built of an inferior variety of sandstone from the Potomac. In England sandstone is much more used for building than other rocks. Edifices of the 12th century, of the hard grits of the coal measures and underlying formations, as Melrose abbey and the cathedral of Glasgow, are in the finest state of preservation; and in some of those of the next century, as Ecclestone abbey near Barnard castle, the original sharp outlines of the delicate mouldings and other decorations are still finely retained; while other edifices, as Durham castle, and even the Hunterian museum in Glasgow, built in 1804, manifest decided symptoms of decay.
The cause of these differences may be the imperfect consolidation of the grains in the poorer kinds, and a texture that admits the absorption of water, which, freezing and thawing within the mass, throws off successive portions from the outside; or it may be the original intermixture of foreign substances that are acted upon by atmospheric influences, as iron pyrites and carbonate of lime. Both of these together are particularly destructive from the sulphuric acid generated in the decomposition of the former attacking the carbonate and removing this from the stone. - The sandstones employed for architectural purposes in the United States are chiefly from quarries of the new red sandstone formation in the Connecticut river valley and in New Jersey; they are also imported from Nova Scotia and New Brunswick, and from Caen in France. The Nova Scotia and New Brunswick sandstones are chiefly from the vicinity of Shepo-dy bay at the head of the bay of Fundy, and are known in the New York market as the Dorchester and Albert stone. The rock is yellowish brown, darker than the Caen stone, of even grain, and much of it very free from foreign substances. The quarries furnish very large blocks, and are directly on the shore of the bay, accessible to large vessels.
For flagging stones several varieties of sandstone answer an excellent purpose, as for example the broad slabs of the Potsdam sandstone already referred to. New York city is chiefly supplied with them from Ulster, Greene, and Albany counties, and from the formation known as the Hamilton group. The principal shipping points are Kingston, Saugerties, Maiden, New Baltimore, and Coxsackie on the Hudson river, and the quantities sent down annually amount to several million square feet. The stone is obtained in immense sheets of any desired thickness from nearly horizontal strata, and is regularly divided by perpendicular joints, which are as smooth as if cut by a saw.