During recent years much attention has been given to the recognition of rock-structures which have been produced by the slumping and sliding of bedded deposits on the sea-floor during or soon after their accumulation, but as it is often difficult to distinguish between structures so formed and those which are the result of tectonic movements which occurred long subsequent to the deposition and consolidation of the rocks affected, it is my purpose to record certain minor bedding-structures which clearly owe their origin to such later tectonic movements in the hope that structures of this origin may more readily be distinguished from those produced by slumping. It is true that the occurrence of movements parallel to the bedding is suspected to have been widespread, and, among others, Boswell, Cobbold, and Greenly have recorded examples from this country (1937, 1904, 1919), yet, because of the general scarcity of direct evidence whereby the magnitude and frequency of such movements can be proved, it would seem to be of importance to describe the structures when the evidence of their later origin is manifest. It is for this reason, and with a view to supplementing the very little information on this topic usually contained in works on structural geology, that it has been thought advisable to make known some of the details of the minor bedding-structures which are visible in particularly instructive outcrops in the cliffs and on the foreshore of the coast of North Cardiganshire.

In the lower parts of its course a river deposits material derived from the upper parts, this deposition being due to loss of velocity. As a deduction it seems to follow that this alluvial deposit (using the term “alluvial” in a rather wide sense) should occur as two strongly contrasted types as regards size of particles.

Judd was one of the early workers to recognize that changes in the mineral constitution of an intrusive igneous rock, changes affecting the earlier minerals, may take place prior to the completion of crystallization. He points out that minerals, since their first crystallization, may have undergone several series of changes, totally dissimilar in kind, and resulting from causes altogether different, and that the dissolved material may be carried away from a crystal and deposited within the cavities of neighbouring crystals of different species. This process must result in the blending together in the most inextricable manner of material derived from different crystallized minerals, and the whole characters of the rock may be completely altered.

In recent years the stratigraphical position of the Craighead Limestone, as assigned by Lapworth (1882) and by Peach and Home (1899), has been questioned by such competent geologists as Ulrich (1930), O. T.Jones (1935), and Cowper Reed (1935); any new evidence having a bearing on the age of the limestone is therefore worthy of being placed on record.

Secondary silicification of country rock is commonly observed in many geological environments, and it is generally considered that this metasomatism was effected by hydrothermal siliceous solutions permeating the host rock. Silicification may be localized in comparatively narrow zones alongside the channelway used by the solutions, or it may result in a regional alteration of a large mass of rock. The former type of occurrence is commonly observed alongside veins of presumed hydro-thermal origin (1, 2) whilst the latter is sometimes seen in rocks, e.g. silicified limestones (1, 3) with no evidence of a path followed by the solutions. In such cases a mechanism of sub-capillary soaking must be assumed; the solutions may act selectively by favouring certain bands or types of rock.

The genus Cummingella was erected by Reed (1942, p. 653) with the genotype “Entomolithus (Oniscites) derbiensis Martin”; the accompanying generic diagnosis, however, was based, not on the description and illustrations given by Martin (1809), but on the concept of Phillipsia derbiensis Auctorum which has prevailed since, at least, the time of H. Woodward's 1883 account published in the Monograph of the British Carboniferous Trilobites. The purpose of this note is to indicate and to attempt to clarify the unsatisfactory nomenclatorial state of both Martin's species and Phillipsia derbiensis Auctorum.

The remarkable specimen here described was collected in 1938 by Mr. W. S. Pitcher during an excursion of the Chelsea Polytechnic Geological Field Club to Yorkshire, and was presented by him to the British Museum in December of that year. The specimen was found in a stone in a dry-wall, but there is little room for doubt that it came originally from the immediate vicinity, for the matrix can be matched with part of the D1 limestones exposed there.