The area described in this paper is about 5 miles east of Rhayader and 4 miles north of Llandrindod Wells. The igneous rocks come to the surface in two localities, namely Baxter's Bank and Caerfagu Mill. The former is approximately half a mile north of the main road from Rhayader to Pen-y-Bont, whilst the latter is situated immediately to the south of the main road and about three-quarters of a mile south-east of the village of Nantmel.

Gypsum or selenite is one of the commonest “decomposition” minerals found in connexion with the exposed surfaces of the Tertiary rocks in southern Ecuador. With the possible exception of the coarser grits and conglomerates, the normal sandstones are usually veined extensively by sheets of this mineral; it also occurs in a fibrous form (plates), in agglomerations of minute scalenohedrons, flake-like folia parallel to the bedding, and also in beautiful rosette-like or “stellate” groupings. In the typical clay shales or “gredas” the distribution of gypsum is not so general, though crystals or irregular groups of imperfectly shaped crystals occur on the weathered slopes or screes. In many localities, where a steep talus slope of shale forms the foot of a superimposing sandstone cliff, the scree itself is invariably covered with broken plates of gypsum in such profusion that from a distance it has the appearance of irregular masses of ice or glass. The platy form (which usually consists of parallel laminae with vertical fibres) is probably the most common and, in reality, each plate has been formed after the manner of a typical vein deposit.

Since the publication of the paper by the present author in 1905 on the Classification of the Phacopidae, a considerable advance has been made in our knowledge of this family as a result of further and better material being obtained, and of new discoveries in different parts of the world. Many new genera and subgenera have been instituted, and modifications or limitations of some of the old terms have been introduced by various authors. The work of Wedekind, Clarke, Rud. and E. Richter, and Kozlowski has specially dealt with questions of classification, but there is still a considerable amount of diversity in the usage and application of the generic and subgeneric names.

In 1925 my friend Mr. M. P. Latter published in this magazine a short paper on the rocks in the neighbourhood of Cardigan. I was interested in these rocks at the time he was working on them, and in his absence abroad I have recently paid a few brief visits to this region; and it may be desirable to place on record such supplementary facts and inferences as have been obtained.

Mr. E. J. Wayland, of the Geological Survey of Uganda, has recently found three interesting fossils which throw further light on the age of the beds to the west and north of Mombasa. One of them is a cast of an ammonite in brown ironstone, which has been kindly examined by Dr. L. F. Spath; he says that being only a cast its specific identification is perhaps impossible. It is a Grossouvria, and he says it is very similar to several examples of G. lateralis (Waagen), from the Callovian anceps beds of Kachh. He identifies it as certainly a member of Siemiradzki's form-series of Perisphinctes balinensis, Neumayr. This identification is of interest as the fossil was obtained at the point 10/6 miles from Mombasa on the Uganda Railway. This position lies to the west of, and therefore at a lower horizon than, the ammonite-bearing shales near Mombasa. At 11 1/9 miles along the railway I found a lamellibranch in some dark shales and Dacqué (Geol. Rundsch., vol. i, 1910, p. 159) has recorded from near by some obscure Cephalopod and plant remains which he identified as Bathonian.

Very little is known of the early history of the non-marine Gastropoda; hence the discovery in the coal-bearing strata of Scremerston (Northumberland) of a small species closely resembling certain modern fluviatile operculate shells is of considerable interest. My thanks are due to Mr. J. Pringle for bringing the specimens in question to my notice; to Mr. R. G. Carruthers, to whom their discovery is due, for sending me particulars; and to Dr. F. L. Kitchin for permission to borrow them from the Museum of Practical Geology, where they are deposited.