Splitting, crushing and spalling which occur when boulders move against each other, generate fine material and so give rise to angular large and small sizes. The nature of the rounding process can be deduced from the analysis of moments of the roundness frequency distribution of the size range 4–128 mm. A low standard deviation indicates the dominance of one process—either abrasion or breakdown; a high standard deviation indicates the presence of both. The relationship between mean roundness, standard deviation of roundness, skewness of roundness and mean size allows for an analysis of the size reduction processes at work on the sediment; and for a comparison between the reactions of different rocks. A model, based on actual data, is given for the evolution of roundness of limestone clasts on a beach.

Tectonic implications of 32 new potassium-argon age determinations of minerals and rocks of Singhbhum and adjacent regions are discussed along with those of recent structural, stratigraphic and petrologic studies, and some pre-existing geochronologic data. Three distinct orogenic cycles with closing dates at c. 3200, 2700 and 850 m.y. respectively have been recognized. The earliest cycle is represented by the Older Metamorphic Group of metasediments with associated basic intrusives and biotite granodiorite gneiss; they occur mainly as relics within the huge Singhbhum granitic complex. Emplacement of the latter (c. 2700 m.y.) marks the culmination of the Iron Ore orogenic cycle which is characterized by the low-grade metamorphic Iron Ore Group sediments and volcanics and associated basic intrusives. Along the northern and eastern edges of the stabilized Iron Ore orogenic belt, thick sedimentation of the Singhbhum and Gangpur Groups (?2000–1700 m.y.) was followed by large scale basic volcanic activity (1700–1600 m.y.); these rocks were folded and regionally metamorphosed in several phases between c. 1550 and c. 850 m.y. (Singhbhum orogenic cycle); extensive thrusting and granitization occurred in the later part of the cycle.

The complex can be divided into three units, here named the Lower, Middle and Upper Layered Series respectively, on the basis of petrochemical data and correction of the dips of the layering for the effects of post-consolidational flexuring. There are no chilled margins, and the contact with the country rock consists of granular basic and ultrabasic rocks, older than the cumulates of the layered series. The intrusive mechanism is considered in the light of field and chemical data.

Inclusion textures within garnets are often used to demonstrate the relationship in time between garnet growth and deformation. The evidence for syntectonic growth is discussed, and it is shown that several thin sections are required before an accurate interpretation can be made. The patterns predicted by Powell & Treagus (1967) are matched with natural examples, and a mechanism for the aevelopment of these fabrics is suggested.

A correlation of the predominantly limestone deposits of the Bathonian (Middle Jurassic) stage in Central England is presented on the basis of the available ammonites. This is related to the standard sequence of Bathonian zones, which are briefly discussed in the introduction. All ammonites from the area of stratigraphic importance are listed. There is nothing to suggest that any of the zones are absent because of erosion or nondeposition, except in the East Midlands at the base of the Bathonian.

The structures are comparable with stromatolites seen in modern “flats” environments and tufa occurring in joints in the granite—gneiss basement is compared with modern and fossil forms. Finely laminated (? varved) carbonates with a large lateral persistency (1: 50,000) are referred to periods of high water level and are believed to show evidence of seasonal algal bloom. The presence of widespread stromatolite structures is consistent with earlier environmental interpretations of the Orkney rocks.