OF late years, owing to the great volume of original papers sent in to the Geological Magazine, it has not been possible or desirable to devote space to the proceedings of the British Association, as was the custom under the editorship of the late Dr. Henry Woodward; but it is manifestly impossible to pass over in silence the celebration of its Centenary, which has just been held in London, under the presidency of the Right Honourable General J. C. Smuts. The personality of the President alone would be sufficient to mark this off as a special occasion, but from the point of view of the geologist there is much more in it than this.

It appears that the geophysical approach to the problems of crustal dynamics leads to a theory of mountain formation that covers the main features of the formation of nappes and is in general agreement with other geological evidence. The chief novelty is in the recognition of the importance of the spreading under their own weight of the rocks uplifted as a result of crustal shortening.

IN the interior of Africa, in the high region of the Portuguese colony of Angola, in localities lying at an altitude of from 3,000–4,000 feet, where the soil consists of a thick layer of sand, clearly derived from the disintegration of the underlying sandstones, one constantly meets with peculiar and interesting out-crops of iron ore in the form of brown ironstone (limonite) of a spongy texture. This brown ironstone has in some places a greyish metallic colour, in places a reddish-brown, brown, brownish-yellow, or other tint characteristic of this hydrated iron oxide, but in structure it is different from all other varieties of this kind of iron ore. The entire mass is full of irregular cavities exactly like those in an ordinary toilet sponge or in pumices, and gives the impression of a ferruginous mass inflated with gases. It also recalls the unforged iron produced in the old-fashioned methods of “burning out” metallic iron direct from the ore used by ancient metallurgists.

IN the neighbourhood of Haddo House, between Methlick and Ellon, some 20 miles north of Aberdeen, there is a considerable body of gabbro that has been named the Haddo Mass. This intrusion is later than the general metamorphism of the Dalradian country-rocks and, from its resemblance to certain dated intrusions, earlier than the local Middle Old Red Sandstone. It belongs, therefore, to the Younger Gabbro portion of the Post-Movement Phase in the igneous history of North-East Scotland.

From the conclusions regarding the formation of complex pegmatites from the field and microscopic evidence, the following sequence of events is suggested:—

Towards the end of the solidification of a granite magma a liquid remains, having the general composition of a common pegmatite, but with small quantities of sodium and some rarer elements. By squeezing during crystallization this magma is separated into two distinct parts, one of which forms common aplite dykes. The other, more aqueous portion, when tapped by fissures, solidifies more slowly to form dykes, sills, or stringers of common pegmatite. During this solidification the albite (in solution), together with rare elements and their compounds, is concentrated in the remaining liquid portion, which now contains a very high proportion of water. This aqueous solution begins to migrate up the dykes or sills of common pegmatite, possibly even before the latter has completely solidified, showing a preference for the foot-wall, and begins to replace the earlier minerals, The first stage is the replacement of potash felspar, either microcline or orthoclase, by albite. Later there is a replacement of albite and sometimes of earlier minerals by spodumene, lepidolite, cassiterite, tantalite, etc. Cassiterite, possibly due to being carried in combination with a volatile element tends to be deposited in the uppermost part of the dyke or sill, where, however, some albite replacement has always preceded it.