Rocks and Their Origins

by Grenville A. J. Cole

THE CEMENTING OF SANDS

The cement of sandstones is very varied. On our modern coasts, springs draining from a limestone land, or even running through banks of broken shells, will deposit calcite in the interstices of the beach, until slabs and shelves of conglomerate and sandstone arise in defiance of the waves. On coasts where calcium bicarbonate is abundant, it may be precipitated by any cause that diminishes its solvent. Mere evaporation, and the escape of carbon dioxide from the water as it is scattered into spray, lead to the deposition of a cement between the grains of sand. As Linck[6] shows, calcite is thus laid down in temperate waters, while aragonite forms fibrous crystals between the detrital fragments on the flanks of tropic isles. Aragonite may also arise from the action of ammonium carbonate or sodium carbonate on calcium sulphate or calcium chloride in sea-water. Sands thus become cemented by one or other form of calcium carbonate. They include, moreover, calcareous algæ, foraminifera, and fragments of coral and sea-shells.

Fossil shells are usually represented in older sandstones by mere external and internal moulds. The texture of the rock allows of their being dissolved in percolating waters, while in clays belonging to the same geological series they may be exquisitely preserved.

In shallows, and especially in lakes, where soluble salts of iron become readily oxidised, brown iron rust, the mineral limonite, is continually forming at the surface and sinking to the bottom, where it firmly cements the sand. A group of bacteria[29] extracts iron in this form from the water of freshwater lakes and swamps, and greatly aids in its accumulation. Though a red colour may appear also in marine deposits, masses of red and purple conglomerates and sandstones may reasonably be assigned a freshwater origin. Such rocks are usually found to be devoid of marine fossils, and they often contain traces of land plants.

Barytes (barium sulphate), which sometimes occurs in veins simulating those of calcite, is an occasional cement of sandstone, evidently arising from subterranean waters.

Bands of flint (chert) occur in certain sandstones, such as the Hythe Beds of the English Lower Greensand Series. These are due to the cementing of certain layers by chalcedonic silica, and the source of this silica is seen in the hollow moulds of sponge-spicules, and the glauconitic casts of their canals, that commonly remain. G. J. Hinde[30] shows that in the Cretaceous examples, as in so many other flints, the majority of the spicules are of the tetractinellid type.

Under arid conditions, as in parts of Africa, loose superficial sands may become cemented by calcium carbonate, or even by silica, brought up in water rising by capillary action from below.

The sand-dunes of the coast of our own islands, which cannot remain wet for long, become in places toughened by a deposit of calcite derived from the abundant shells of land-snails. In the Cape of Good Hope[31] the dunes, as A. W. Rogers states, are converted by invasions of calcium carbonate, "into hard rock through a distance of many feet from the surface, and where repeatedly wetted and dried, as happens where the sea has encroached upon old dunes, the rock becomes intensely hard and weathers with a peculiarly jagged surface." The General Post Office and the South African Museum in Cape Town are mainly constructed of this recently consolidated rock.

The modern sandstones cemented by silica are still more interesting. In the Cape of Good Hope, and notably in the Kalahari desert, they form the intensely hard rock known as Quartzite[32]. The cementing material is true quartz, which sometimes deposits itself in bipyramidal crystals about the grains of sand. The molecules of such crystals are arranged in continuity with the grouping of those in the original detrital grain, as is proved in thin sections under the microscope by the optical continuity of the quartz of the grain and of its coating. As silica continues to be deposited, the coatings interlock, and the rock passes into true quartzite. It is now often difficult to detect the outline of the original grains. Such superficial quartzites may be ten feet thick at most, with uncemented sand below. Rogers suggests that the cementing process may have originated in shallow pools; but it has obvious analogies with that which forms iron-pans and superficial masses of calcium carbonate in regions where capillary waters are subject to prolonged evaporation. H. G. Lyons[33] has attributed the cementing of parts of the Nubian Sandstone in the desert of Lower Egypt to the silica set free by the alteration of the felspars in the rock. This change, he suggests, was accelerated by the infiltration of sodium carbonate of local origin. Fossil trees in these strata have been replaced by silica. A further example is recorded by Armitage[34] from Victoria, where friable ferruginous Cainozoic sands have been converted into quartzite. This type of rock, the hardest known, and associated in our minds with high antiquity and metamorphic action, proves, then, to be in process of construction at the surface at the present day.

The observations of Rogers show that quartz and not mere chalcedony is deposited on the grains of sand. The "crystalline sandstones" of Permian and Triassic age in England may, then, have acquired their remarkable characters at the actual epoch of their accumulation. This is rendered the more probable by the recognised occurrence of arid conditions, at any rate seasonally, when the strata in question were laid down.