Abstract
LONDON
Geological Society, June 21.1-Prof. P. Martin Duncan, F.R.S., president, in the chair—12. On the mechanism of production of volcanic dykes and on those of Monte Somma, by R. Mallet, F.R.S. The author stated that in 1864 he made a careful trigonometrical survey of the escarpment of Monte Somma, especially with reference to the numerous dykes by which the rocks composing it are intersected. He described in detail the phenomena of direction of trie dykes, especially as regards the axis of the cone of Vesuvius; to this direction he gives the name of orientation. Of twenty-seven dykes ten presented an approximately vertical line, whilst all the rest had a sensible dip or “hade.” The dykes are in no cases intersected by coherent beds of lava, but in one instance the top of a dyke was stopped by such a bed. Many of the dykes bifurcated or branched, and frequently two dykes intersected each other at considerable angles. These and other circumstances prove that the dykes were produced at different and successive ages. Many of them were fractured and displaced in consequence of movements of the mass of rock traversed by them; and these dislocations are regarded by the author as indicating the vast extent and force of the internal movements, due principally to gravity, which are constantly taking place in the mass of volcanic cones. These movements greatly influence the position of the dykes, and render it difficult to ascertain that which they originally occupied. The dykes thin out at various heights, and their superior and northern terminations were found not to reach the existing surface, notwithstanding the amount of denudation that has taken place; and hence the author concludes that they never reached the surface of Somma, when it was the wall of an active volcano. The author further indicated a process by which beds or plates of lava descending the slopes of a volcano may change their direction, and becoming embedded in the detritus accompanying or following them, may, to a greater or less extent, simulate dykes, although in this case the two sides of the plate will present the differences always seen in the upper and under surfaces of a bed of lava. The orientation-lines of five or six of the observed dykes were said to pass approximately through the axis of the cone of Vesuvius, but all the rest presented great diversities, and some, when prolonged, would not touch the eone at all. In making a lithological examination of the dykes of Somma, the author directed particular attention to the position of the elongated air-bubbles found in the material of each dyke, considering that the direction of the longest axis of these bubbles would indicate the flow of the material when in fusion. He stated that on the whole the long axes of the bubbles are nearly horizontal or pointing at moderate angles upwards in directions very nearly parallel to the plane of the dykes at the place where they occur. Hence he inferred that the dykes were filled by injection not from below but nearly horizontally. The author further referred to the mineralogical characters of the materials of the dykes, and stated that they are not all composed of leucitic lava; he also mentioned the occurrence of cross columnar structure in some of the larger ones. After referring to the differences Observable in the physical condition of the two surfaces of some dykes, the author proceeded to consider the mode of origin of the fissures, which, when filled, constitute volcanic dykes. He maintained that the production of a fissure and its filling with molten matter must have been simultaneous and due to the same cause, namely, the hydrostatic pressure of the liquid lava more or less filling the crater, the pressure originating the fissure into which the pressing liquid at the same time enters; a fissure thus produced and filled will always be widest near the crater, so that if the material of the cone were perfectly uniform the dykes produced will be wedge-shaped. But from the absence of this uniformity and other causes, fissures commenced at the interior and propagated into the mass of volcanic cones can rarely be uniformly distributed round the crater or produced in regular vertical planes in a truly radial direction. Hence the author concluded that it is unsafe to attempt to fix the position of an ancient crater by means of the intersection or concurrence of the lines of apparent orientation of dykes alone. The author stated that the intrusion of volcanic dykes cannot so greatly influence the slope of volcanic mountains as has been supposed.—13. On the metamorphic rocks surrounding the Land's End mass of granite, by S. Allport. In this paper the author described the results of a microscopic examination of certain metamorphic rocks surrounding the Land's End granite, indicating the changes produced by the intrusion of the latter upon clay slate and upon certain igneous rocks. The slates in contact with granite become converted into tourmaHne-and mica-schists, and are found to contain crystalline quartz, tourmaline, and three distinct varieties of mica, with occasionally tremalite, magnetite (and andalusite?), and in some localities felspar. Their structure is also changed, the most remarkable changes being foliation with every gradation from nearly straight parallel lines to the most complicated contortions, and concretionary structure by segregation of quartz and mica, the result being a spotted schist. With regard to the origin of the granite of Cornwall, the author said that neither observation in the field nor microscopical study lends any support to the notion that it is a metamorphic rock; but, on the contrary, that there is the clearest evidence of former deep-seated volcanic action in the disturbance and alteration described in his paper, and in the enormous number of granitic and felsitic dykes intersecting the country for miles. The mode of occurrence of granite in other localities also seems to him to furnish evidence in the same direction.—14. On the relation of the upper carboniferous strata of Shropshire and Denbighshire to beds usually described as Permian, by D. C. Davies. The author stated his conviction that from the Spirorbis-limestone upwards to and including the Permian we have one continuous series of deposits.—15. Notes on the physical geography and geology of North Gippsland, Victoria, by A. W. Howitt. The earliest formation of which any trace is left in this district is the silurian, all traces of any older rocks being removed, probably by the same agencies which have contorted and metamorphosed the silurian slates and sandstones. The surface of all these silurian strata show signs of great denudation previous to the deposition of the Devonian. The period that elapsed between these two epochs was one of volcanic activity, apparently sub-aerial and terrestrial, and representing the Lower Devonian. The Middle Devonian strata consist of shales and sandstones devoid of any traces of volcanic action, which, however, again becomes apparent in the Upper Devonian. The latter consists of conglomerates, sandstones, and shales, interstratified with aqueous deposits. The prevailing red colour of these beds the author suggests may possibly indicate lacustrine rather than marine conditions. The next in the series of deposits present in North Gippsland are of Tertiary age, and rest horizontally on the flanks of the mountains at elevations nowhere exceeding 1,000 feet. At the close of the Miocene and at the commencement of the Pliocene periods the land probably was from 300 feet to 400 feet lower than at present. The fact that different genera of fish are found in the streams flowing from the north and south sides of the Australian Alps indicate the high antiquity of that watershed. These mountains have been formed by the gradual elevation of the land en masse, and its equally gradual erosion by the streams and rivers.—16. Further notes on the Diamond Fields, &., of South Africa, by E. J. Dunn. Communicated by Prof. A. C. Ramsay, F. R. S. These notes are intended to serve as additions and corrections to the author's paper read in 1873.—17. On Chesil Beach, Dorsetshire, and Cahore Shingle Beach, co. Wexford, by G. H. Kinahan, M.R.I.A., &. Communicated by Prof. Rarnsay, F.R.S., V.P.G. S. The author carefully conipares the situations, structures, &., of these two shingle beaches, and points out that their wonderful similarity is due to nearly the same natural Causes in each case, but that at Chesil the driftage is due to the flow-tide current augmented by waves caused by the prevailing winds, while at Cahore the driftage is solely due to the flow-tide currents, its effects being modified by adverse wind-waves. The sorting of the pebbles on Chesil Beach is probably chiefly caused by the progressive increase in the velocity of the tidal current as it approaches the nodal or hinge-line of the tide in the English Channel. The author considers that the current due to the flow of the tide has greater drifting powers than wind-waves.—18. Some recent sections near Nottingham, by the Rev. A. Irving, B.A. The author describes'a section of the strata exposed during the recent, construction of a railway line from Carlton, three miles to the east of Nottingham, through Daybrook, to Kimberley.—19. On the permians of the north-east of England and their relations to the under- and overlying formations, by E. Wilson. The author describes the same section as that noticed in the preceding paper.—20. The section at high force, Teesdale, by C. T. Clough.—21. The distribution of flint in the chalk of Yorkshire, by J. R. Mortimer, communicated by W. “Whittaker. The author considers that the present shape of the Chalk Wolds of Yorkshire seems to suggest that they are the remains of an atoll or circular reef, probably one of a chain, rather than the fragment of a vast sheet of cretaceous mud deposited in deep water. He thinks that the flint-bearing and non-flint-bearing chalk areas are in the main contemporaneous in Yorkshire. The chalk without flint contains 4.28 per cent, of silica, whilst the chalk with flint contains only 2.12 per cent.—22. On the mode of occurrence and derivation of beds of drifted coal near Corwen, North Wales, by D. Mackintosh.—23. The Cephalopoda-beds of Gloucester, Dorset, and Somerset, by J. Buckman.—24. Evidence of the subsidence of the Island of Guernsey, by R. A. Peacock, C. E. All round the coast of this island, like that of Jersey, are found tree-trunks and other vestiges of old forest-land now submerged. Passages are quoted by the author from various old historians relative to the former existence of this tract as dry land, the submergence of which probably took place in the fifteenth century. The encroachment of the waters is due to the subsidence of the land, and not, as has been suggested, to the breaking in of the sea through some natural barrier upon some already low-lying district. Judging from the old chart of 1406, the amount of depression is equal to 160 feet.
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Societies and Academies . Nature 14, 302–304 (1876). https://doi.org/10.1038/014302a0
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DOI: https://doi.org/10.1038/014302a0