The mineralogical changes in green schists and related quartzofelspathic schists of sedimentary origin are discussed, and the following conclusions are reached as to the conditions of formation of oligoclase in these rocks:—

(1) Oligoclase normally appears as a product of dynamothermal metamorphism at relatively high grades such as prevail in the zones of almandine and perhaps kyanite. It is accompanied either by deeply-coloured hornblende, hornblende and biotite, or biotite and muscovite, according to the chemical composition of the rocks in which it occurs.

(2) Sodic oligoclase containing 10 per cent to 15 per cent of anorthite may occur with pale aluminous hornblende in green schists lying within the more strongly metamorphosed portion of the chlorite zone. The rocks in question are low in potash and have been formed by reconstitution, at a higher grade, of chlorite-epidotealbite-schists containing calcite. This oligoclase-hornblende association is not to be confused with the actinolite-epidote-albite-chlorite assemblage which is formed at any grade within the zone of chlorite, by direct reconstitution of basic igneous rocks without change in bulk composition and in the absence of CO2. A slight modification of Tilley’s subdivision of the green schist facies of Eskola is therefore introduced.

(3) A zone of oligoclase representing a grade of metamorphism higher than that attained in the biotite zone, may be recognized for quartzo-felspathic schists of appropriate composition and for many green schists, in areas of progressive regional metamorphism. In the latter case, blue-green hornblende often accompanied by biotite is also present.

(4) Oligoclase or more calcic plagioclase and deeply-coloured hornblende form readily during purely thermal metamorphism of only medium grade in the absence of stress. This accounts for the irregular distribution of both these minerals in districts where purely thermal and regional metamorphism have both occurred during the same period of orogeny.

While investigating the problem of coastal changes in South Wales, and examining 1 the area of the Merthyr Mawr Warren (Fig. 1) for this purpose, the differences, observable in the surface conditions to-day, and evident in the records of the past, between the areas east and west of a line approximating to the Burrows Well stream, suggested that the relatively recent physical history of the two sections of the area had not been similar. The greater width in the east suggested recent accumulation in this section, and the presence of what appeared to be successively formed littoral dunes pointed to a relatively recent phase of prograding, while the topography made it possible that these changes were related to the mouth of the River Ogmore.

Tinto hill by reason of its height, its massive but shapely form, and its splendid isolation, dominates the whole of middle Clydesdale. It forms an east-west ridge of tadpole-like outline—massive and lofty towards the eastern end and tailing away towards the western extremity, the only point at which it makes contact with the neighbouring uplands. On the east it rises directly from Clydesdale, where its lower slopes pass into a drift-mantled bench, which is from 1 to 2 miles in width. The lower and flatter portions of this bench are covered by a sheet of sand and gravel 30 or more feet above the Clyde flood-plain. The gravel spread extends into the broad open valleys which isolate Tinto to north and south, and is there undergoing dissection by the Glade Burn and Garf Water. The dissection is, however, hardly more than begun, and the undulating and sometimes moundy character of the surface of the gravel sheet reflects the uneven nature of the original deposition. Rising sharply from this surface are certain striking mounds and ridges which have received some mention in geological literature under the name of the Thankerton Kames. They were first noted by Sir Archibald Geikie (2, p. 44),1 and later described, with a map indicating their extent and distribution, by Professor Gregory (3, p. 474). They have been further discussed by Professor Charlesworth (1, pp. 31–37), who is unable to accept Professor Gregory’s interpretation of the facts. More recently they have been mapped by the officers of the Geological Survey (4), together with the better-known Kames of Carstairs, and in this connection a note by Dr. M. Macgregor on the latter (6) is of some interest.