the conclusion is drawn that the hypothesis of corrosion is as inapplicable to the latter as to the former. On the contrary, the author believes that the cavities of the spherulites are the result of the hydrous state of the magma,

March 20th.-J. J. H. Teall, Esq., M.A., V.P.R.S., President, in the Chair.

The following communications were read :

1. On a Remarkable Volcanic Vent of Tertiary Age in the Island of Arran, enclosing Mesozoic Fossiliferous Rocks.'

Part I. On the Geological Structure.' By Benjamin Neeve Peach, Esq., F.R.S.L. & E., F.G.S., and William Gunn, Esq., F.G.S.

The rocks which form the subject of this paper cover an area of about 7 or 8 square miles, and culminate in Ard Bheinn A'Chruach and Beinn Bhreac. They are in contact with formations ranging from the Lower Old Red Sandstone to the Trias, and are later in date even than the important faults of the area. They are made up partly of fragmental volcanic materials, and partly of various intrusive masses, confined within an almost unbroken ring of intrusive rocks. In addition to igneous fragments, the clastic volcanic rocks contain fragments derived from the surrounding formations; and also masses of shale, marl, limestone, and sandstone belonging to formations not now found in situ in the island. One of these is several acres in extent, contains fossils, and is in part of Rhætic age; a second is a fragment of Lias; and a third is of limestone and chert resembling the Antrim Cretaceous rocks, and yielding fossils. The absence of Oolitic and older Cretaceous seems to indicate a resemblance between a former succession in Arran and that now seen in Antrim. If these fragments fell into the vent from above, the igneous rocks must be of post-Cretaceous age, and they give an impressive picture of the amount of denudation which has occurred since the period of vulcanicity.

Part II. Palæontological Notes.'

By E. T. Newton, Esq., F.R.S., F.L.S., F.G.S.

The masses of Rhætic strata yield Avicula contorta, Pecten valoniensis, Schizodus (Axinus) cloacinus, etc.; those of Lower Lias Gryphaa arcuata, Ammonites angulatus, and new species of Nuculana and Tancredia, which are figured and described. Thin slices of the Cretaceous limestones prove to be very like those of the Antrim Chalk, and the rocks yield determinable foraminifera, Inocerami, sponges, and echinoderms.

2. On the Character of the Upper Coal Measures of North Staffordshire, Denbighshire, South Staffordshire, and Nottinghamshire; and their Relation to the Productive Series.' By Walcot Gibson, Esq., F.G.S.

The Upper Coal Measures of North Staffordshire are capable of

a fourfold subdivision, the groups representing a definite sequence of red and grey strata :

4. The Keele Series. Red and purple sandstones and marls with occasional seams of coal, and bands of entomostracan limestone.

3. The Newcastle-under-Lyme Series. Grey sandstones and shales, with four thin seams of coal, and at the base an entomostracan limestone.

2. The Etruria Marl Series. Mottled red-and-purple marls and clays, with thin green grits; a thin coal occurs 150 yards above the base.

1. Black Band Series. Grey sandstones, marls, and clays; numerous thin seams of coal and Blackband ironstone; one of many thin bands of limestone is constant, 36 to 40 feet above the base.

Spirorbis- and entomostracan limestones attain a maximum in the Upper Coal Measures, but are not unknown in the productive measures below. Indeed the two sets of measures are closely allied lithologically, palæontologically, and stratigraphically in this region. The chief movements are pre-Triassic and post-Carboniferous.

No attempt has been made to recognize the Black Band Series in the Denbighshire, South Staffordshire, and Nottinghamshire coalfields, as they are indistinguishable from the productive measures in the absence of Blackband ironstones. In each of these areas there are divisions in the Upper Coal Measures which correspond with the three highest divisions in North Staffordshire, and in all cases, except near the margin of the basin, where overlap occurs, they are underlain by ordinary Coal Measures with coal-seams. It is therefore concluded that these higher Coal Measures were deposited in one basin which included all the four areas dealt with, and that whatever movements occurred were of a local, and not of a regional character. Judging by published descriptions, the higher series of Measures appear to be present in other Midland and North-western coalfields, and in most of them the Keele Series corresponds to the Salopian Permian of Prof. Hull.

April 3rd.-Horace W. Monckton, Esq., F.L.S., Vice-President,

in the Chair.

The following communication was read :—

The Igneous Rocks and Associated Sedimentary Beds of the Tortworth Inlier.' By Prof. Conwy Lloyd Morgan, F.R.S., F.G.S., and Sidney Hugh Reynolds, Esq., M.A., F.G.S.

It has long been known that igneous rocks occur in the district under consideration, but opinions are divided as to their intrusive or contemporaneous character. Evidence is here brought forward to show that the igneous rocks form two bands, the lower inter bedded with Upper Llandovery strata, and the upper overlain by Wenlock, and that both bands are probably contemporaneous lavas. The igneous rocks appear at two horizons, both in the Charfield Green district and also in the district which includes Avening

Green, Damery, Micklewood, Daniel's Wood, etc. At Charfield their general run is north-north-west and south-south-east, and the upper band is associated with a bed of calcareous ash. The ash contains lapilli, felspar-crystals, quartz-grains, small shaly patches, and fossils, cemented by calcareous matter. The fossils, determined by Mr. Cowper Reed, probably indicate the Wenlock age of the rock. The associated trap would thus seem to be interbedded -a conclusion strengthened by its uneven surface and highly amygdaloidal

character.

At Daniel's Wood the higher bed of trap is overlain by limestones which contain Wenlock fossils, and underlain by rocks with Upper Llandovery fossils. The dip of the rocks appears to indicate the existence of an anticline. The rocks underlying the trap-band of Damery Quarry are not seen, but above the trap are rocks bearing Upper Llandovery fossils. This trap occupies a large area near Woodford Farm. The same band of trap at Middle Mill underlies an ash-bed in which fossils of Upper Llandovery age have been found. The rocks, as a whole, follow the north-eastern and northern boundaries of the Bristol Coalfield.

The microscopic examination of the lower igneous rock shows that it is a basaltic andesite containing plagioclase (acid andesine or oligoclase), pseudomorphs after enstatite, with chloritic and ironoxide patches. The higher bed sometimes contains fresh augite, and both bands frequently contain rounded grains of quartz. In other examples the felspars appear in three forms, with augite and enstatite, and the rock ranges from an andesite to a porphyritic basalt. The quartz-grains present appear to be xenoliths. The silica-percentage of the rocks on a moisture-free basis varies from 61 to 67, while the specific gravities are from 2-74 to 2.99.

XI. Intelligence and Miscellaneous Articles.

To the Editors of the Philosophical Magazine.

GENTLEMEN,

Battersea Polytechnic, London, S. W.

IN N the June number of the Phil. Mag. there appears a letter from W. Rollins commenting on a paper of mine in your issue for April. It appears to me to be entirely unnecessary; as nowhere in my paper have I said that he, originally, was responsible for the idea that the gas escapes through the walls of a Crookes tube. If he had read my paper before writing his letter he would probably have noticed this; and, if we may judge from the abstract of his own papers which he is good enough to give, he would perhaps have found other points of interest in the paper not noticed in his own.

I am, Gentlemen,

Yours faithfully,

R. S. WILLOWS.

XII. On Ether and Gravitational Matter through Infinite
Space. By Lord KELVIN *.

[This is an amplification of Lecture XVI., Baltimore, Oct. 15, 1884, now being prepared for print in a volume on Molecular Dynamics and the Wave Theory of Light, which I hope may be published within a year from the present time.]

NOTE ON THE POSSIBLE DENSITY OF THE LUMINIFEROUS MEDIUM, AND ON THE MECHANICAL VALUE OF A CUBIC MILET OF SUNLIGHT.

[From Edin. Royal Soc. Trans. vol. xxi. Part I. May, 1854; Phil. Mag. ix. 1854; Comptes Rendus, xxxix. Sept. 1854; Art. lxvii. of Math, and Phys. Papers.]

THAT

§ 1. THAT there must be a medium forming a continuous material communication throughout space to the remotest visible body is a fundamental assumption in the undulatory Theory of Light. Whether or not this medium is (as appears § to me most probable) a continuation of our * Communicated by the Author.

+ [Note of Dec. 22, 1892. The brain-wasting perversity of the insular inertia which still condemns British Engineers to reckonings of miles and yards and feet and inches and grains and pounds and ounces and acres is curiously illustrated by the title and numerical resuits of this article as originally published.]

[Oct. 13, 1899. In the present reproduction, as part of my Lec. XVI. of Baltimore, 1884, I suggest cubic kilometre instead of "cubic mile" in the title, and use the French metrical system exclusively in the article.] § [Oct. 13, 1899.-Not so now. I did not in 1854 know the kinetic theory of gases.]

Phil. Mag. S. 6. Vol. 2. No. 8. Aug. 1901.

M

own atmosphere, its existence is a fact that cannot be questioned when the overwhelming evidence in favour of the undulatory theory is considered; and the investigation of its properties in every possible way becomes an object of the greatest interest. A first question would naturally occur, What is the absolute density of the luminiferous ether in any part of space? I am not aware of any attempt having hitherto been made to answer this question, and the present state of science does not in fact afford sufficient data. It has, however, occurred to me that we may assign an inferior limit to the density of the luminiferous medium in interplanetary space by considering the mechanical value of sunlight as deduced in preceding communications to the Royal Society [Trans. R. S. E.; Mechanical Energies of the Solar System; republished as Art. LXVI. of Math, and Phys. Papers] from Pouillet's data on solar radiation, and Joule's mechanical equivalent of the thermal unit. Thus the value of solar radiation per second per square centimetre at the earth's distance from the sun, estimated at 1235 cm.-grams, is the same as the mechanical value of sunlight in the luminiferous medium through a space of as many cubic centimetres as the number of linear centimetres of propagation of light per second. Hence the mechanical value of the whole energy, kinetic and potential, of the disturbance kept up in the space of a cubic centimetre at the earth's distance from the sun*, is 1235 412

or of a cm.-gram.

3 × 1010' 1010

§ 2. The mechanical value of a cubic kilometre of sunlight is consequently 412 metre-kilograms, equivalent to the work of one horse-power for 5.4 seconds. This result may give some idea of the actual amount of mechanical energy of the luminiferous motions and forces within our own atmosphere. Merely to commence the illumination of eleven cubic kilometres requires an amount of work equal to that of a horse-power for a minute; the same amount of energy exists in that space as long as light continues to traverse it; and if the source of light be suddenly stopped, must pass from it before the illumination ceases t. The matter which possesses this energy

The mechanical value of sunlight in any space near the sun's surface must be greater than in an equal space at the earth's distance, in the ratio of the square of the earth's distance to the square of the sun's radius, that is, in the ratio of 46,000 to 1 nearly. The mechanical value of a cubic centimetre of sunlight near the sun must, therefore, be 1235 × 46,000 or about 0019 of a cm.-gram.

3×10:0

Similarly we find 4140 horse-power for a minute as the amount of work required to generate the energy existing in a cubic kilometre of light near the sun.