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Usually they occur as massive or nodular limestones, underlaid by a fine shale or flag-stone; and in other cases, as in the noted Denbighshire sandstones, as a coarse grit of very great thickness. This grit forms mountain ranges through North and South Wales, and is generally marked by the great sterility of the soil where it occurs.
Of these, the siliceous are chiefly made up of sand or flinty grains; the argillaceous, or clayey, of a mixture of siliceous matter with a certain proportion, about a fourth in weight, of aluminous earth; and, lastly, the calcareous rocks, or limestones, of carbonic acid and lime.
Many other minerals may be inclosed in limestones in small quantities, and an exact qualitative analysis will often show the presence of sulphides or sulphates, phosphates, and titanates, and the alkali or even the heavy metals. No attempt is made in the following procedures to provide a complete quantitative scheme which would take into account all of these constituents.
No abysmal deposits have been found, and the limestones of the period are those which would be laid in clear, warm water of moderate depth like that of modern coral seas. The shales of the Utica and Hudson show that the waters of the sea now became clouded with mud washed in from land.
Most limestones contain some percentage of magnesium, and in the case of a pure dolomite over 45% carbonate is present in combination with calcium carbonate. A stone rich in magnesium slakes less readily than one high in calcium, and therefore is preferred by manufacturers shipping pulverized burnt lime to reach its destination before slaking. Relative Values.
Clay-slate may be altered shale, and granular marble may have originated in the form of ordinary limestone, replete with shells and corals, which have since been obliterated; and, lastly, calcareous sands and marls may have been changed into impure crystalline limestones.
The white and green marls of the Headon series, and some of the accompanying limestones, often resemble the Eocene strata of France in mineral character and colour in so striking a manner as to suggest the idea that the sediment was derived from the same region or produced contemporaneously under very similar geographical circumstances. Solenastraea cellulosa, Duncan.
Again, whilst Bain found fossil plants in his reptiliferous strata north of the Cape, and Livingstone and Thornton discovered coal in sandstone, with fossil plants, like those of our old coal of Europe and America, yet both these mesozoic and palaeozoic remains are terrestrial, and are not associated with marine limestones, indicative of those oscillations of the land which are so common in other countries.
Along with minor oscillations there were laid in the interior sea various formations of sandstones, shales, and limestones, and from Kansas to South Dakota beds of white chalk show that the clear, warm waters swarmed at times with foraminiferal life whose disintegrating microscopic shells accumulated in this rare deposit.
No shells or corals have ever been found in the whole series, not even where the beds are calcareous, forming irregular courses of concretionary lumps called "corn-stones," which may be described as mottled red and green earthy limestones.
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