by compression the air remains to a large extent in the vapour above the liquid and dissolves but slowly in the liquid formed; consequently, as the volume of vapour diminishes the undissolved air becomes more and more compressed, and its pressure increases and becomes considerable when the vapour is nearly all condensed. Similarly, with an impurity more volatile than the pure substance, a partial "fractionation" of the liquid takes place, the less volatile pure substance condensing first, whilst the vapour becomes more and more contaminated with the more volatile impurity.

On the other hand, when the readings are taken with increasing volume, the air or volatile impurity is at first uniformly distributed through the liquid, and it escapes but slowly through the long column of liquid into the vapour above. Thus when a reading is taken with a relatively small volume of vapour, the air present has a much greater influence in the first case than the second, but when the volume of vapour is relatively large its influence is small in both cases. I have noticed repeatedly that the freer a liquid is from air or permanent gas, the more closely do the readings during compression and expansion agree.

The presence of a small quantity of impurity less volatile than the pure substance may escape detection so long as the volume of liquid, relatively to that of the vapour, is fairly large; but its influence becomes very marked when the quantity of liquid is relatively very small. Thus, on one occasion, the isopentane had come in contact for a moment with an indiarubber tube and had, no doubt, dissolved a minute quantity of it. It was noticed in this case that the mercury was slightly tarnished (owing in all probability to the sulphur in the indiarubber), but the vapour-pressures, when there was plenty of liquid present, agreed among themselves and with previous determinations. When, however, the volume was increased until nearly the whole of the liquid was evaporated, the observed vapour-pressures were far too low, and diminished rapidly as the volume was increased.

In this case and in those in which the presence of any permanent gas was detected, the experimental results were rejected and the tube was refilled.

LXVIII. Proceedings of Learned Societies.

GEOLOGICAL SOCIETY.

[Continued from p. 504.]

June 6th, 1894.-Dr. Henry Woodward, F.R.S., President, in the Chair.

THE following communications were read :—

THE

1. On the Banded Structure of some Tertiary Gabbros in the Isle of Skye.' By Sir Archibald Geikie, LL.D., D.Sc., F.R.S., F.G.S., and J. J. H. Teall, Esq., M.A., F.R.S., Sec.G.S.

Four

After calling attention to the previous references to the pseudobedding and banding of the gabbro-masses of the Inner Hebrides, the authors describe the rocks which form the rugged ridge of Druim-an-Eidhne, near the head of Glen Sligachan. This ridge is made up of parallel beds, sheets, or sills disposed in a general N.N.W. direction with a prevalent easterly dip. distinct types of gabbro occur:-(1) dark, fine-grained, granulitic gabbros; (2) well-banded gabbros; (3) coarse-grained massive gabbros; and (4) pale veins of a highly felspathic gabbro. The relative ages of the banded and granulitic gabbros have not been definitely settled; but the coarse, massive gabbros are certainly intrusive in the banded series and the pale veins cut all the other varieties.

The paper deals mainly with the banded gabbros. They occur in successive sheets or sills which vary from a few feet to many yards in thickness, and consist of parallel layers of lighter and darker material which correspond in direction with the trend of the sheets, and are usually inclined to the east or south-east at angles ranging from 20° to 30°. In some cases the bands can be seen to have been puckered or folded.

The minerals entering into the composition of the banded, as also of the other varieties, are labradorite, pyroxene, olivine, and titaniferous magnetite. The banding is due to a variation in the relative proportions of the different constituents and especially in the amount of magnetite. Some narrow bands and lenticles are composed entirely of pyroxene and magnetite. The variations in chemical composition are illustrated by three analyses by Mr. Player. The microscopic characters of the rocks are described, and it is shown that the minerals of the banded gabbros have not been crushed or broken since they were formed.

The authors conclude that the banding is the result of the intrusion of a heterogeneous magma and that similar banding in certain portions of the Lewisian gneiss may have been produced in the same

way.

2. On the Microscopical Structure of the Derbyshire Carboniferous Dolerites and Tuffs.' By H. H. Arnold-Bemrose, Esq., M.A., F.G.S.

The paper deals with the petrography of the Toadstones or igneous rocks of Derbyshire. Brief reference is made to the work of previous petrographers, the age of the rocks, and the question as to the number of beds. The outcrops mapped by the Geological Survey, and several additional ones, have been examined, and the results given in a table for the purpose of the paper and for future reference.

The Toadstone is divided into massive rocks or lavas, and fragmental rocks or tuffs. The former consist of olivine-dolerite, either with granular or with ophitic augite, and olivine-basalt. The rock is often very fresh, but in some places is altered to a diabase. The principal constituent minerals are described. A pseudomorph of olivine, optically like biotite and somewhat like Iddingsite but differing from it chemically, is fully described.

The latter portion of the paper deals with the tuffs, which are much more extensive than has been hitherto supposed. Specimens are described, taken from thirteen outcrops.

3. On the Origin of the Permian Breccias of the Midlands, and a Comparison of them with the Upper Carboniferous Glacial Deposits of India and Australia.' By R. D. Oldham, Esq., F.G.S.

The author first describes the Permian Breccias of the Midland Counties of England, which he had the opportunity of examining at Eastertide of the present year. He describes the characters of the Breccias, and concludes that they were formed subaërially as gravelfans by rivers charged with a maximum load of sediment, and therefore incapable of performing any appreciable amount of erosion, An examination of many of the fragments at Abberley and some at Church Hill reveals the presence of scratches, which occur in such a manner that the author believes they existed on the fragments before they were transported, and discusses the evidence for their production by ice or soil-cap movement, deciding in favour of the former.

A short description of the Upper Carboniferous deposits of India follows, and it is pointed out that they differ markedly from the deposits of Britain. Amongst other things the separation of different pebbles by considerable interspace of matrix, and the bending of stratification-planes round a pebble as though the pebble had dropped from above, is noted, and it is maintained that floating ice alone will account for these pebbles being dropped into the Indian deposits. Finally, it is remarked that the so-called Upper Carboniferous deposits of India and the Permian deposits of the Midlands of Britain may be practically contemporaneous, as maintained by the late Mr. H. F. Blanford, indicating a possible simultaneous existence of glaciers in England, India, and Australia.

June 20th.-Dr. Henry Woodward, F.R.S., President,

in the Chair.

The following communications were read :—

1. On Deep Borings at Culford and Winkfield, with Notes on those at Ware and Cheshunt.' By W. Whitaker, Esq., B.A., F.R.S., F.G.S., and A. J. Jukes-Browne, Esq., B.A., F.G.S.

The four borings are described in detail, so far as the specimens examined would permit; these were few in the case of Culford, but many from the other borings. The following is an abstract of the formations traversed in each :

The interest of the Culford boring centres in its striking the Paleozoic floor at the small depth of 6371⁄2 feet; but the age of the slaty rocks cannot be determined. Although only 20 miles east of Ely, no Jurassic rocks exist and the Lower Cretaceous series is only about 32 feet thick, the beds differing greatly from those of Cambridgeshire but resembling those of the same age in the Richmond boring.

The Winkfield boring (3 miles W.S.W. of Windsor) is remarkable for having been successful in obtaining water from the Lower Greensand, and for the great depth (1243 feet) to which it was carried for this purpose, the Gault being unusually thick.

The boring at Ware is now for the first time described in detail, and former accounts are corrected from specimens preserved by the New River Company. By this means, and with the assistance of Mr. W. Hill, the authors are able to give a fairly complete account of the rocks and to determine the limits of the divisions of the Upper Cretaceous Series. They deny the existence of Lower Greensand at this locality.

Of the boring at Cheshunt a more complete account is now given, based on information and specimens supplied by Mr. J. Francis, the Engineer of the New River Company.

The paper concludes with a tabular view of all the borings in the East of England, showing the level below Ordnance datum at which the Paleozoic floor occurs in each.