pole southward over both continents at the same time, causing cyclonic conditions in the Atlantic both in summer and winter. Such a condition of things would have flooded Western Europe with warm southerly winds. No such meteorological difficulties arise if the hypothesis that the more important glacial and interglacial periods alternated in the western and eastern continents be adopted. Thus persistent and excessive cold in North America during the winter of 1898-99 was coincident with abnormal warmth in Europe; the winds were northerly and polar in America, southerly and strictly complementary in Europe. On the other hand, the effect of an ice-sheet anticyclone extending from Greenland to Central Europe might have been to force the storm-tracks of the North Atlantic to the south-west, producing warm south-easterly winds in Labrador, which would have tended, moreover, to divert the surface-currents of the North Atlantic from the European to the American coast. The glaciation of Great Britain could only have happened at a time when the IcelandoBritish Channel was closed. No permanent ice-sheet could have existed in Britain and Scandinavia while the influence of the Gulfstream was as it is at present. It is possible that the shifting of glacial conditions from one side of the Atlantic to the other may have been due to differential earth-movements. The views taken in this paper afford a simpler explanation of geological facts than those usually adopted. Instead of supposing that the climatic changes of the Great Ice Age, several times recurrent at intervals of a few thousand years, were due to astronomical or physical causes, it is suggested that the climate of the northern hemisphere being, from some unexplained cause, colder than that of our era, conditions of comparative warmth or cold may have been more or less local, affecting the great continental areas at different periods. May 22nd.-J. J. H. Teall, Esq., M.A., V.P.R.S., President, 1. On the Skull of a Chiru-like Antelope from the Ossiferous Deposits of Hundes (Tibet).' By Richard Lydekker, Esq. 2. On the Occurrence of Silurian (?) Rocks in Forfarshire and Kincardineshire along the Eastern Border of the Highlands.' By George Barrow, Esq., F.G.S. These rocks occur in three lenticular strips between the schistose rocks of the Highlands and the boundary-fault next the Old Red Sandstone. The largest is about 20 miles long, and extends almost from Cortachy to beyond the Clattering Bridge; it is about mile wide at its widest. The rocks are divided into two groups: the Jasper and Green-Rock Series below and the younger Margie Series above. A section along the North-Esk River is described in detail, and other sections referred to it. The lower division consists of fine-grained sandstones (bearing microcline), grey slaty shales, jaspers (sometimes containing circular bodies resembling radiolaria), and a variable series of basic igneous rocks (green rock') of coarse texture and probably intrusive origin. The upper division consists of conglomerates, pebbly grits, dark and white shales, pebbly limestone, and grey shale. The age of the series cannot be definitely ascertained, but the lower division is compared with the Arenig cherts, etc. of the Southern Uplands, while the Margie Series is newer than this, but older than the Old Red Sandstone. Both groups have been much deformed, but the sediments contain clastic micas and have undergone practically no recrystallization, and the igneous rocks are never changed into hornblende-schists. The deformation is greatest near the junction with the Highland Schists, giving rise to a deceptive appearance of an upward succession and an apparent transition in crystalline character, but the crushing never extends more than a few yards into the Highland Series. A major thrust separates the Highland Schists from the Jasper and Green-Rock Series, and a minor thrust generally separates the latter from the Margie Series. The position of the major thrust and that of the later great boundary-fault skirting the Old Red Sandstone have been determined by the outer limit of the aureole of crystallization of which the South-Eastern Highlands form a part. The harder crystalline schists to the north-west have snapped off from the softer portions, now covered by newer rocks to the south-east. 3. 'On the Crush-Conglomerates of Argyllshire. By J. B. Hill, Esq., R.N. While the sedimentary origin of the Highland Boulder-bed is proved by the foreign boulders contained in it, there occur in the Loch-Awe region certain conglomerates, often along definite horizons, which may have been confused with it, but which the author is able to prove have originated by crushing. The sedimentary rocks of the area include all the members of the LochAwe Series, consisting of grits, slates, and limestones, the latter being mostly gritty in character. Associated with these is an enormous amount of igneous material of Dalradian age, ranging from intermediate to basic in composition, together with porphyrite-dykes probably of Old-Red-Sandstone age, and a plexus of Tertiary dykes. The sediments are everywhere folded, the folds being of isoclinal type. The Dalradian igneous material consists of epidiorites; and evidence is brought forward to prove that these rocks are intrusive, while their great apparent bulk is probably to be accounted for by repetition due to folding. A petrographical description is given of the various types of rocks represented among the epidiorites, the minerals of which include hornblende and felspar, with chlorite, epidote, calcite, quartz, and iron-ores. There is every gradation in texture from a coarse gabbro-like type to the finest schists, and some of the rocks are vesicular. The rocks are frequently foliated. The crush-conglomerates have been observed in the limestones, quartzites, and epidiorites; but they are most conspicuously developed at the junction of rocks of dissimilar character, and especially when the limestone and epidiorite are in juxtaposition. The junction of the two rocks is intricately folded: folded knobs of epidiorite measuring from a few inches to a foot or more being packed together in a limestone-matrix. In the sections big blocks may be seen in process of division by shearing-movements, which have succeeded the folding. The limestone seems generally to have played the part of a plastic body, and has accommodated itself as a matrix to the folded and isolated fragments of epidiorite, between which it has been squeezed. Thus the origin of the conglomerate is satisfactorily proved by the fact that it contains fragments of rocks newer than the sediments in which the crush-conglomerates are embedded. The author considers that it would be safer to regard such conglomerates in this area as have a calcareous matrix as having been formed by crushing. June 5th.-J. J. H. Teall, Esq., M.A., V.P.R.S., President, in the Chair. The following communications were read :— 1. 'On the Passage of a Seam of Coal into a Seam of Dolomite.' By Aubrey Strahan, Esq., M.A., F.G.S. The author was informed by Mr. N. R. Griffith in 1900 that the Seven-Feet Seam of the Wirral Colliery had been found to pass into stone of an unusual character. For a distance of 1600 yards from the shaft this seam was good, and about 4 feet thick. A little farther in, bands of stone from 1 to 10 inches thick made their appearance in it, and, gradually increasing in thickness, these bands eventually constituted the whole seam, the last traces of workable coal disappearing at 250 yards from the point where the change first began. The boundary of the barren area has been found for a distance of 1480 yards, and it runs north and south. The stone is at first black, but after weathering it becomes grey, and displays curious structures, among which are pisolitic, or mammillated structures, the intervening spaces being filled with coaly matter. One specimen displays woody tissue filled with dolomite. Analyses by Dr. W. Pollard yield from 18.5 to 13 per cent. of magnesia. The phenomena are not those of a wash-out,' as there is no sign of erosion, but there is proof that the dolomite was formed in almost motionless water, and the conditions appear to have been those under which a tufa would form. It appears to have been formed on a spot to which clastic material scarcely gained access, and which was reached even by vegetable matter in scant quantity and in a finely divided condition. 2. On some Landslips in Boulder-Clay near Scarborough.' By Horace W. Monckton, Esq., F.L.S., V.P.G.S. In 1893 Mr. Clement Reid drew attention to a foliated structure developed in Drift at Beeston, near Cromer (Proc. Geol. Assoc. vol. xiii, p. 66), and soon afterwards the present author noticed examples of a very similar character in Boulder-Clay on the Yorkshire coast. The Clay forms much of the cliffs, and slips, large and small, are very frequent. When the Clay is dry, vertical cracks forming a sort of columnar structure occur, and the Clay breaks away in lumps, while a moister condition causes flow, producing more or less horizontal flow-structure which, as in the Cromer case, has the appearance of irregular bedding. The author illustrated his remarks by photographs of the cliffs taken by himself. LVII. On the Induction-Coil. By Lord RAYLEIGH, F.R.S.* ALTHOUGH several valuable papers relating to this subject have recently been published by Oberbeck †, Walter, Mizuno §, Beattie, and Klingelfuss T, it can hardly be said that the action of the instrument is well understood. Perhaps the best proof of this assertion is to be found in the fact that, so far as I am aware, there is no a priori calculation, determining from the data of construction and the value of the primary current, even the order of magnitude of the length of the secondary spark. I need hardly explain that I am speaking here (and throughout this paper) of an induction-coil working by a break of the primary circuit, not of a transformer in which the primary circuit, remaining unbroken, is supplied with a continuously varying alternating current. The complications presented by an actual coil depend, or may depend, upon several causes. Among these we may enumerate the departure of the iron from theoretical behaviour, whether due to circumferential eddy-currents or to a failure of proportionality between magnetism and magnetizing force. A second, and a very important, complication has its origin * Communicated by the Author from the Jubilee volume presented to Prof. Bosscha. + Wied. Ann. lxii. p. 109 (1897); lxiv. p. 193 (1898). Wied. Ann. lxii. p. 300 (1897); lxvi. p. 623 (1898). § Phil. Mag xlv. p. 447 (1898). || Phil. Mag. 1. p. 139 (1900). Phil. Mag. S. 6. Vol. 2. No. 12. Dec. 1901. 2 Q in the manner of break, which usually occupies too long a time, or at least departs too much from the ideal of an instantaneous abolition of the primary current. A third complication arises from the capacity of the secondary coil, in virtue of which the currents need not be equal at all parts of the length, even at the same moment of time. If we ignore these complications, treating the break as instantaneous, the iron as ideal, and the secondary as closed and without capacity, the theory, as formulated by Maxwell*, is very simple. In his notation, if a, y denote the primary and secondary currents, L, M, N the coefficients of self and mutual induction, the energy of the field is If c be the primary current before the break, the secondary current at time t after the break has the expression The S being the resistance of the secondary circuit. current begins with a value c. M/N, and gradually disappears. The formation of the above initial current is best understood in the light of Kelvin's theorem, as explained by mein an early paper t. For this purpose it is more convenient to consider the reversed phenomenon, viz., the instantaneous establishment of a primary current c. The theorem teaches that subject to the condition a=c the kinetic energy (1) is to be made a minimum; so that Mc+Ny=0 gives the initial secondary current. In the case of the break we have merely to reverse the sign of y. Immediately after the break, when a=0 and y has the above value, the kinetic energy is Immediately before the break the kinetic energy is Le2, so that the loss of energy at break-the energy of the primary spark-is LN-M2 (3) "Electromagnetic Field," Phil. Trans. 1864; Maxwell's Scientific Papers, i. p. 546. "On some Electromagnetic Phenomena considered in connexion with the Dynamical Theory," Phil. Mag. xxxviii. p. 1 (1869); Scientific Papers, i. p. 6. |