In tracing beds of sandstone across a tract of country, it is frequently found that the character of the strata gradually alters; mud or clay becomes associated with the sandy deposit, until finally the sandstone is replaced by beds of dark coloured shale. Similarly the sandy detritus on the ocean floor, or in an inland lake, when followed further and further from the source from which the materials were derived, passes by degrees into argillaceous sand, and finally into sheets of dark clay or mud. The hardened beds of clay or fine grained mud become transformed into SHALES. As a general rule, then, shales are rocks which have been laid down in places further from the land, or at a greater distance from the source of origin of the detrital material, than sandstones or conglomerates. The conglomerates, or old shingle beaches, usually occur in somewhat irregular patches, marking old shore-lines or the head of a river delta. Coarse sandstones, or grits, may occur in the form of regularly bedded strata stretching over a wide area; and shales or clays may be followed through a considerable extent of country. The finer material composing the clays and shales has been held longer in suspension and deposited in deeper water in widespread and fairly horizontal layers. In some districts sandstones occur in which the individual grains show a well marked rounding of the angles, and in which fossils are extremely rare or entirely absent. The close resemblance of such deposits to modern desert sands suggests a similar method of formation; and there can be no doubt that in some instances there have been preserved the wind-worn desert sands of former ages. Aeolian or wind-formed accumulations, although by no means common, are of sufficient importance to be mentioned as illustrating a certain type of rock. [Sidenote: CALCAREOUS ROCKS.] The thick masses of limestone which form so prominent a feature in parts of England and Ireland, have been formed in a manner different from that to which sandstones and shales owe their origin. On the floor of a clear sea, too far from land to receive any water-borne sediment, there is usually in process of formation a mass of calcareous material, which in a later age may rise above the surface of the water as chalk or LIMESTONE. Those organisms living in the sea, which are enclosed either wholly or in part by calcareous shells, are agents of limestone-building; their shells constantly accumulating on the floor of the sea give rise in course of time to a thick mass of sediment, composed in great part of carbonate of lime. Some of the shells in such a deposit may retain their original form, the calcareous body may on the other hand be broken up into minute fragments which are still recognisable with the help of a microscope, or the shells and other hard parts may be dissolved or disintegrated beyond recognition, leaving nothing in the calcareous sediment to indicate its method of formation.