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The preservation of molluscan shells as calcite casts is commonly a result of their centripetal replacement, while on the sea floor, by a stable micnte envelope. This grows in three stages: (1) algae bore into the sheU wall, (2) the algal filaments die and decay, (3) micritic aragonite fills the tubes. Later, if the skeletal sand is exposed to fresh water as a result of uplift or a fall in sea level, or to a suitable brine deep under the sea floor, the residual molluscan shell is dissolved but the envelope remains intact. It forms an empty mould that is subsequently filled with drusy calcite. Complete replacement of skeletal grains by micritic aragonite yields cryptocrystalline grains like Illing's "grains of aragonite matrix" and the grains in Beales's "bahamite". Through a study of compaction fabrics it may be possible to decide whether an envelope-bearing limestone was cemented above sea level or below the sea floor.

Diagenetic fabrics (textures and structures) have been examined in thin sections of Dinantian limestones mainly from the Avon Gorge, North Wales and Yorkshire. Six processes are shown to have been responsible for the change from unconsolidated sediment to limestone. These are: (1) granular cementation and drusy growth, (2) rim cementation (secondary enlargement), (3) pressure solution, (4) grain growth sensu stricto, (5) mechanical deposition in cavities of post‐depositional age, and (6) post‐depositional formation of cavities by erosion and solution in a carbonate mud. Grain growth, a process well known to metallurgists, acts in monomineralic fabrics of low porosity in the solid state. The intergranular boundaries wander so that some grains enlarge while others shrink and disappear, so bringing about a general increase in coarseness. In all the limestones the mosaic between the mechanically deposited particles (skeletal debris, pellets, ooliths) is one of three types. These are granular cement, rim‐cemented detrital crystals, and a mosaic caused by grain growth.

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A field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the U.K. Lithologies include grainstones, packstones, wackestones and lime mudstones. The rocks are of Silurian, Carboniferous and Jurassic ages. Additional information was obtained from other limestones in the U.K., the U.S.A., Canada and continental Europe. The beds are parts of sequences composed of couplets of strata, fissile limestones alternating with hard limestones. In the fissile limestones the effects of mechanical compaction and pressure‐dissolution have been concentrated, whereas in the hard limestones they are minimal or absent. Bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe. The features produced by pressure‐dissolution are dissolution seams and fitted fabric: there are no stylolites. The original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed. The vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent. It is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed. Thenceforth, mechanical compaction and then pressure‐dissolution were concentrated in the less cemented strata: these became the fissile limestones. Pressure‐dissolution acted late in the diagenetic history and provided only an insignificant part, if any, of the carbonate for cementation. It is concluded that the orientation of beds (couplets) is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously. Similar couplets in platform limestones of the Mississippian and Pennsylvanian of the U.S.A. extend over thousands of square kilometres. The signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected. The presumed order of events was (1) accumulation of carbonate sediment, terrigenous clay and organic matter, (2) hydrodynamic reworking and bioturbation. the latter finally overprinting the former, (3) selective cementation of the more carbonate‐rich strata yielding couplets, each consisting of a relatively well‐cemented stratum and a poorly cemented stratum, (4) mechanical compaction concentrated in the less cemented strata, (5) pressure‐dissolution concentrated in the same strata.