The subject of this address is a brief acount of the succession, thickness, and geological history of the South African, and more especially of the Transvaal, formations. The information necessary for such an account is of course very incomplete, but in broad outline the succession is now known, and some speculation as to the physical conditions that prevailed during the building up of the region may perhaps be permitted. I propose to deal with the period of the geological history of this country that came to an end with the close of Karroo times. The Karroo period ends with the Stormberg rocks (Rhæitic), and since that time South Africa has, with the exception of a small coastal area, been a land surface, and the rocks have consequently been exposed uninterruptedly to the forces of denudation.

A Recent visit to Portugal has given me an opportunity of seeing the geological features of the neighbourhood of Lisbon and the Lower Tagus; and though of a somewhat cursory nature I venture to bring them before the Society, being supplemented by reference to the investigations of previous observers, and a study of the excellent geological maps which are in the library of this Society. I make no pretension to be an original explorer, but I hope to be able to show that there is a very interesting physical history, not hitherto written so far as I am able to discover, bringing down the account of the changes which have taken place from the Cretaceous period to the Recent, or Quaternary, epoch.

Distribution.—Recent: Red Sea (Klunzinger). Pleistocene, fossil: ? Jebel Zeit (Gregory, 1898); also Old Beach, Camp 7; Wadi Barud, Eastern Egypt. No. J 1789.

That silica appears as a constituent mineral of igneous rocks in two distinct phases, viz. quartz and tridymite, has been known for some time. The writer is not aware that any explanation has been offered which would indicate the conditions determining the appearance of one or the other in a cooling fused rock-mass. He now proposes to describe certain experiments made with a view to explain why it is that though the free silica generally appears as quartz, yet occasionally, as in certain trachytes, it crystallizes out as tridymite.

In estimating the maximum load which pressed upon the northern type of Glacial (Dwyka) Conglomerate in Prieska, Cape Colony, I assumed that the calculations of Sir Wyville Thomson and Bernacci were correct, and that the greatest column of ice that could exist on the earth's surface was from 1,400 to 1,600 feet high. This limit, however, is by no means accepted by European glacialists, who, though they do not go as far as Dr. Croll in assuming thicknesses of 120,000 feet, yet see no reason why there could not have been ice-sheets 5,000 feet thick. The publication of Captain Scott's narrative of the voyage of the “Discovery” has given us certain definite data from the Antarctic which enable the case for the 1,600 feet maximum to be put with more confidence, and I will endeavour in the present paper to state the main lines of the argument. The question is of importance not only to us in South Africa with our two Palæozoic ice-ages, but to all geologists, as it affects the problem of the earth's equilibrium. To give a recent example, Professor Penck, in describing the Bodensee, discusses whether the weight of ice pouring down from the Alps in a sheet 3,600 feet thick may not have had some effect in producing a sinking in the earth's crust.

The very uniform trend of all these ridges will have been noted, but my previous remarks on the general strike of the foreshore chalk require considerable modification. The whole of the chalk so far exposed may be divided into four sections. Each of these comprises an exposure in or close to the cliff of what appears to be the highest part of a ridge running down the beach in a direction from 10° to 30° south of east (and sinking as it goes) to about the half-tide level. Here three of them (the exception being the brickfield chalk) turn and run for some way roughly parallel to the shoreline, and then resume their original direction and run out to sea. The brickfield chalk only varies from this plan by running out to sea with practically no change of direction on the way. Except where a ridge is running up to the cliff, the substratum of the beach is invariably glacial clay down to about half-tide level. Here it is either banked against what appear to be vertical faces of chalk or else (between the foreshore exposures.) disappears under the sand. It has never been seen to run out to sea, and every time a fresh bit of the foreshore below the half-tide level is cleared of sand it is chalk that is revealed. In the case of the section attached to the north bluff there is below the half-tide level a continuous mass of chalk with perfectly regular bedding exposed for at least 1,000 yards along the shore, and directly opposite the north bluff I have myself seen chalk continuous from the foot of the bluff for over 200 yards straight out to sea (except about 20 yards close up to the bluff, which, however, are covered by Mr. B. B. Woodward's letter in the October (1905) number of the GeologicalMagazine), the regular sequence of the beds being only broken by one fault. The section attached to the ridge coated with a sheet of flint is a good second in size, showing chalk in regular sequence for a length of over 400 yards, and maximum (exposed) breadth of about 45 yards.

The chemical classification of igneous rocks is rendered difficult by the large number of substances which are present in them. H. S. Washington, who based his system of classification upon the composition of standard rock-forming minerals, found it necessary in his great work to divide his 2,880 rocks into no less than 167 final groups in order to ensure a close proximity between the rocks within each group.