My interest in cleat was first aroused by the study of the Geological Survey memoir on the coals of South Wales (1908), from which it became clear to me that the subject of the origin of anthracite was intimately bound up with that of cleat. The object of the present communication is, however, not directly connected with the anthracite question, upon which I hope to have something to say when researches upon which I am engaged with Mr. E. J. Edwards, M.Sc., of Cardiff, have reached a more advanced stage.

The island of South Georgia is nine hundred miles S. 80° E. from the Falkland Islands. The coastline of the island is rock-bound and more or less precipitous, and there is neither open beach nor detrital flat on its outward margin. A narrow valley, a morainic flat, inside Cumberland Bay, runs up 4 miles to a glacier, and there are other patches of morainic material, marking the recession of the glaciers. Leith Harbour is one of these, and Elsie and Adventure harbours, originally enlarged by the ice into one channel, have been separated into two safe anchorages by morainic material.

Cumberland Bay Series: Upper Division.—This consists largely of hard, coarse, compacted crystal tuffs, dark-grey or black in hand specimens, and resembling coarse grey wackes. In thin section they contain fragments of trachytic lavas, and detached crystals of orthoclase and oligoclase in a paste of minutely crystalline material mixed with much indeterminate matter. Iron-ores and ferromagnesian minerals are rare. In some of the rocks andesitic and latitic fragments are dominant, and occasional fragments of yellowish glass enclosing euhedral orthoclase are seen. Many of the rocks have suffered some degree of scapolitization. The tuffs pass into siliceous grey wackes still with some recognizable volcanic material, and are interbedded with fine banded mudstones which appear to represent volcanic muds. Radiolaria have been recognized in these rocks as well as in the tuffs. The mudstones occasionally become somewhat laminated and pass into black shales. An imperfect cleavage traverses many of these rocks at a high angle to the bedding. These tuffs doubtless represent the interbedded ‘diabas - schalstein’ of Thürach.

The special interest of the island of South Georgia depends on the evidence it promises as to the geological history of that part of the Southern Ocean which lies south of the South Atlantic. According to the well-known views of Professor Suess, South Georgia is on a continuation of the mountain line of the Andes, which at the southern end of South America bends eastward along the northern margin of Drake's Sea and continues 30° to the east, where it turns southward; it completes a great horseshoe-shaped course by passing through South Georgia and returning westward through the South Orkneys to Grahamland.

The recent discussion on the origin of the Himalaya, initiated by Colonel S. G. Burrard in his paper “On the Origin of the Himalaya Mountains”, has centred in the theories of isostasy and mountain compensation advocated by the Rev. O. Fisher and Mr. J. F. Hayford.

In his latest contribution to the subject Colonel Burrard states very clearly the problem of isostasy now requiring solution—

“Continents and mountains have been found to be compensated by underlying deficiencies of density; how has this condition resembling hydrostatic equilibrium arisen upon a solid globe of rock?”

In a recent paper I have advanced reasons for believing in the existence in the earth's crust at a certain depth, at present unknown, of a highly garnetiferous shell of rock, for which the name infra-plutonic zone or shell is suggested. This zone is situated at such a depth that pressure becomes a dominant factor in mineral transformations, the accompanying high temperature ensuring a sufficient degree of molecular mobility. It is suggested that under the influence of these high pressures (and temperatures) reactions will ensue between the various ferromagnesian silicates—micas, amphiboles, pyroxenes, olivines—and anorthite felspar, with the formation of garnet as a characteristic mineral: the reason assigned is that a reduction of volume accompanied by an absorption of heat is thereby effected, it being accepted that the garnet-forming reactions are endothermic.

If one can accept the philosophical necessity for the existence of this infra-plutonic shell, then some theory of isostasy seems logically to follow, with the garnetiferous plastic-solid shell as the cushion upon which the isostatic adjustments of the earth's crust have their foundation.

The presence of cordierite does not seem to have been often recorded from the granite of Dartmoor, in spite of the abundance of that mineral amongst the contact-altered sediments surrounding the igneous mass. The recently published Survey memoirs on Dartmoor and Ivybridge cover nearly the whole area of the granite, and I can only find references to two specimens in which cordierite, or rather pinite pseudomorphs representing that mineral, have been detected microscopically. Within the limits of the granite proper the mineral does not appear to have been observed at all in an unaltered condition, at least so I infer from the absence of any record of it in the above-mentioned memoirs, as I must confess to a somewhat scanty knowledge of West of England geological literature. Having recently done a certain amount of geologising along the northern margin of Dartmoor, which has not been investigated by the Survey since the introduction of modern petrographical methods, and having obtained some rather interesting results, of which I hope to publish shortly a detailed account, I was glad to take advantage of an opportunity presented by a visit to the South of Devon to make some comparisons with the contact and other phenomena described by the Survey.

In 1866 I communicated a paper to the Geological Society of London on the Pleistocene Succession in the Trent Basin. The boulderclays and outwash deposits of this district are of two distinct kinds, the one containing rocks from the west and north-west, and the other boulders etc., from the east or north-east of the district. In all cases, except where they have been ploughed up and re-arranged by the ice itself, the drifts containing westerly rocks only are the lowest, and the drifts with easterly rocks have been spread over them. We thus have two distinct ice-flows to deal with.

Section 15 deals with the very interesting subjects of desert films and crusts. Two features are very noticeable in many desert limestone rocks, the production of a hard crust on the outside and of a powdery soft interior. The outer hard layer is often of different colour from the internal portion and highly polished. Professor Walther points out that such hardened crusts occur in buildings in Central Europe, where he suggests that during the dressing of the stone the fine powder produced has been so driven into the pores that chemical agencies are no longer as active. I ventured a suggestion for the production of such sub-crystalline surfaces where much exposed to the sand-blast in which action such as the above was vaguely present to my mind, but whatever external physical effect such external activities may have, the more important is undoubtedly the concentration of solutions near the surface under the influence of the solar heat.