physical conditions unlike those which gave rise to the camptonites and bostonites. In discussing the general laws of differentiation the author points out that it must have taken place before crystallization to any extent had occurred, because there is a marked difference in mineralogical composition between the rocks occurring as bosses and those occurring as dykes; and, further, that it is dependent on the laws which determine the sequence of crystal-building, in so far as the compounds which, on given conditions, would first crystallize are those which have diffused to the cooling margin, and so produced a contact-stratum, of peculiar chemical composition, before any crystallization had taken place. 2. On the Sequence of Perlitic and Spherulitic Structures (a Rejoinder to Criticism).' By Frank Rutley, Esq., F.G.S. This paper relates to the order in which the perlitic and spherulitic structures have been developed in a felsitic lava of Ordovician age from Long Sleddale, Westmoreland. The author having described this rock in a paper, published in the Quarterly Journal of the Society in 1884, and the accuracy of the views then expressed having been questioned, now endeavours to confirm his original statements, adducing in support fresh observations made upon this and other rocks of a similar kind. 3.Enclosures of Quartz in Lava of Stromboli, etc., and the Changes in Composition produced by them.' By Prof. H. J. Johnston-Lavis, M.D., F.G.S. The author describes the existence of enclosures of quartz in a lava-stream at the Punta Petrazza on the east side of Stromboli, and also in the rock of the neck of Strombolicchio. He describes the effects of the rocks upon the enclosures, concluding that the quartz has undergone fluxion but not fusion, and has supplied silica to the containing lavas, thus causing an increase in the amount of pyroxene and a diminution in the amount of magnetite in the portions of those lavas that surround the inclusions and raising the percentage of silica. He suggests that such a process at greater depths and higher temperature may, under certain conditions, convert a basic rock into a more acid one, so that possibly the andesite of Strombolicchio may have been of basaltic character at an earlier period of its progress towards the surface. He offers the suggestion that other rocks or minerals once associated with the quartz have been assimilated by the magma. December 6th, 1893.-W. H. Hudleston, Esq., M.A., F.R.S., The following communications were read: 1. The Purbeck Beds of the Vale of Wardour.' By the Rev. W. R. Andrews, M.A., F.G.S., and A. J. Jukes-Browne, Esq., B.A., F.G.S. The authors have obtained better evidence than previously existed for calculating the thicknesses of the several parts of the Purbeck series in the Vale of Wardour, and comparing the different subdivisions as developed in that Vale with those exposed in other localities. Putting together 22-24 feet of basement-beds of the Lower Purbeck strata seen in the Wockley section, 21 feet in Ridge Quarry, and 17 feet in Teffont Quarry, more than 60 feet of Lower Purbeck Beds are actually seen, and allowing for the gap between the Wockley and Ridge sections, 70 feet seems a fair estimate of the average thickness of the Lower Purbeck strata. In the Teffont and Chicksgrove quarries, a little over 12 feet may be measured between the basal clay of the Middle Purbeck Beds and the Cinder Bed; while the great cutting on the Teffont line shows 19 feet of strata above the Cinder Bed, and the beds of Dinton cutting may be all on a higher horizon; so that an estimated thickness of 32 feet for the Middle Purbeck Beds is probably below the mark. The clay and sand in the Dinton cutting must be 18-20 feet thick, and between its summit and the top of the second seam of calcareous grit there is a thickness of at least 8 feet. This grit forms the floor of Dinton Well, about 40 feet deep; hence there is a thickness of at least 66 feet of Upper Purbeck strata, and probably more than 2 and less than 12 feet in addition. A comparison is instituted between the Purbeck Beds of the Vale of Wardour and those of the Dorset coast, etc., and some remarks are made upon the physical conditions under which the beds were deposited. 2. On a Picrite and other Associated Rocks at Barnton, near Edinburgh.' By Horace W. Monckton, Esq., F.L.S., F.G.S. The object of this paper is to describe a cutting on a new railway in Barnton Park, where there is an excellent exposure of picrite. It consists of serpentinized olivine, augite, mica, iron oxide, and a little plagioclase-felspar, with a variable amount of interstitial matter. In many respects it comes very near to the picrite of Inchcolm, which island is 4 miles north of Barnton cutting. It differs from the picrite of Bathgate, and the probability is that the Barnton rock is an offshoot from the same magma as that which supplied the Inchcolm rock. Besides the picrite other igneous rocks from the same cutting are described-in particular, a rock with porphyritic crystals of a green mineral replacing olivine, or more probably augite, and a great quantity of brown mica in small flakes and crystals. It is suggested that the name of mica-porphyrite might be given to this rock. 2. On a Variety of Ammonites (Stephanoceras) subarmatus, Young, from the Upper Lias of Whitby.' By Horace W. Monckton, Esq., F.L.S., F.G.S. December 20th, 1893.-W. H. Hudleston, Esq., M.A., F.R.S., President, in the Chair. 1. On the Stratigraphical, Lithological, and Palæontological Features of the Gosau Beds of the Gosau District, in the Austrian Salzkammergut.' By Herbert Kynaston, Esq., B.A. This paper, after referring to the previous literature of the subject, treats of the situation and physical aspects of the Gosau Valley, the distribution of the Gosau Beds, their stratigraphy, palæontology, and geological horizon, and the physical conditions under which they were deposited, and a comparison is instituted between the Gosau Beds and the equivalent beds of other areas. The author shows that Hippurites occur at two horizons in the Gosau Beds,—a hippurite-limestone immediately above the basement-conglomerate being characterized essentially by Hippurites cornu-vaccinum, which is overlain by Actœonella- and Nerinaa-limestones and an estuarine series, and above these is a second hippurite-limestone characterized essentially by Hippurites organisans. It is pointed out that Toucas similarly distinguishes two hippurite-zones in Southern France; the lower, characterized essentially by H. cornu-vaccinum, being placed by him at the top of the Turonian System, whilst the second, with H. organisans, is referred to the summit of the Senonian; and the author gives reasons for regarding the Gosau zones as the equivalents of those of the South of France, in which case the Gosau Beds will represent the uppermost Turonian and the whole of the Senonian, i. e., the zones of Holaster planus, Micraster, Marsupites, and Belemnitella mucronata in England, whilst the upper unfossiliferous beds may be the equivalents of the Danian Beds. The strata are, on the whole, of shallow-water origin, and were deposited in shallow bays in the Upper Cretaceous sea of Southern and Central Europe, on the northern flanks of the Eastern Alps. Probably towards the close of Upper Cretaceous times the southern area of the Gosau District was cut off from the sea to form a lakebasin in which the upper unfossiliferous series was deposited. 6 2. Artesian Boring at New Lodge, near Windsor Forest, Berks.' By Prof. Edward Hull, M.A., LL.D., F.R.S., F.G.S. The boring described in this paper was carried down from a level of about 220 feet above Ordnance-datum, through the following beds : London Clay.. feet Lower London Tertiaries Chalk Upper Greensand.. Gault Lower Greensand The Chalk was hard, and contained very little water; but on reaching the Lower Greensand the water rose in the borehole to a height of 7 fect from the surface. The author discusses the probability of the Lower Greensand yielding a plentiful water-supply in the Windsor district. 3. Boring on the Booysen Estate, Witwatersrand,' By D. Telford Edwards, Esq. An account is given of a boring on the Booysen estate, situated about 2 miles from Johannesburg, and about 5000 feet south of the nearest point of outcrop of the Main Reef' of the Witwatersrand. The Bird-Reefs' crop out generally at a distance of 4000 feet south of the Main Reef. The borehole, 1020 feet deep, passed through sandstones (often micaceous), quartzites, and conglomerates, the last-named having a collective thickness of 91 feet 7 inches, the two thickest reefs being respectively 26 and 22 feet thick. The dip of the beds was 35°. Traces of gold were obtained. All the reefs were highly mineralized, principally with iron-pyrites, and belonged to the Bird-Reef' series which overlies the Main Reef. XL. Intelligence and Miscellaneous Articles. MOLECULAR ENERGY OF GASES. To the Editors of the Philosophical Magazine. IT Agreement to 4 figures (or to 7 for that matter) should always be suspected, especially since molecular weights themselves are not known accurately to 3 places. If Dr. Gore is still inclined to attribute importance to his figures, he may convince himself by inventing hypothetical substances with comic molecular weights and finding that they obey his law equally well. As I understand him, however, he writes to request information as to what is known about molecular kinetic energy, A good deal is known; for instance the following:- The mean square of molecular velocity is given for perfect gases by ρ and since o varies as the molecular weight, for different gases at the same temperature, it follows that the average kinetic energies of the molecules of all permanent gases at the same temperature and pressure are the same. But this statement does not hold for imperfect gases or vapours, because for them the external or applied pressure is not expressible so simply in terms of velocity, but is something more like p = }} pu2 - Ap2; where the last term depends upon intermolecular forces, or what Clausius called the Virial, and Laplace denoted by K. Besides this simple matter of separate gases, a much more interesting thing about mixed gases was discovered by Clerk Maxwell in 1860 (January, Phil. Mag.)—namely, that when a number of independent gases are thoroughly mixed in the same box, the average molecular kinetic energy of every variety of molecule is the same. This he found by direct mechanical reasoning, for he showed that at every encounter of unequal elastic spheres any initial or outstanding difference of energy was diminished by the collision; whence, since there are millions of collisions per second in ordinary gases, a condition of average uniformity of distribution of energy is soon attained. From this law of Maxwell's the law of Avogadro, or a slight extension thereof, immediately follows; viz., that in any given volume of such a mixture, the number of molecules of each gas is proportional to that gas's individual pressure. : The proof is as follows:-p=nmu2, mu2 = const., therefore n varies with p; and this holds for every gas separately, provided that there are no intermolecular forces, chemical or other. March 1, 1894. I am, Gentlemen, Your obedient servant, AN EXPERIMENT TO PHOTOGRAPH THE SPECTRUM OF LIGHTNING. BY G. MEYER. In taking a photograph of the spectrum of lightning by means of a spectroscope provided with a slit, the difficulty is met with that, in order to obtain a satisfactory result, the lightning must be exactly in the prolongation of the optical axis of the apparatus. By the following arrangement this limitation may be removed. In front of the object-glass of the photographic apparatus arranged for parallel rays, a glass refraction-grating is fixed. An image of the lightning is then obtained on the plate, produced by the rays which traverse the grating without change of direction, and, on each side of this, images which owe their origin to the rays forming the lateral spectra. The number of images in each lateral spectrum is equal to the number of bright lines occurring in the spectrum of the lightning. During a storm at night in May of this year (1893) I made an experiment of this kind, for which a small hand camera was used; in front of the object-glass (an ordinary landscape-lens of 10 cm. focal distance) a glass grating of 0.0256 mm. breadth of grating, was fixed. Two plates were exposed. The first shows two flashes, and right and left of each of them an image which corresponds to the lateral spectrum of the first order. In the present case this only contains one line. On the second plate were more flashes, and in the immediate neighbourhood of the horizon, above which projected roofs of houses with various chimneys. Between the chimneys a strong flash is to be observed, with its two lateral spectra. I measured the angle |