Abstract
LONDON. Geological Society, December 1.—Prof. W. I. Sollas, F.R.S., president, in the chair.—W. G. Fearnsides: The Tremadoc slates and associated rocks of south-east Carnarvonshire. Results obtained in making a detailed map of the country about Portmadoc, Tremadoc, and Criccieth in Carnarvonshire, and a description of the stratigraphy of the Cambrian and Ordovician rocks there exposed. The sedimentary series are described in the order of their formation. The succession is tabulated. The folding, cleavage, faulting, and jointing of the rocks are described, and an attempt is made to show some relationship between the stress-phenomena which have produced these structures. The great fault through Penmorfa is interpreted as a thrust-plane hading gently to the northeast. It is supposed to form the lowest sole of the group of thrust-planes which follow the southern margin of the Snowdonian mountain-tract. The well-known pisolitic iron ore of Tremadoc is shown to follow the line of this fault. Direct evidence of overthrusting has been got from a study of the graptolite-bearing Llandeilo rocks of Tyddyndicwm, which have been exposed in two artificial trenches dug for the purpose, and the distribution of the andesitic volcanic series in lines of detached lenticles among the Grey Slates is described as evidence of a similar re-duplication of the newer rock-series of the north-eastern district on a more extended scale. It is noted that the dolerites are (1) unaffected by cleavage and faulting, and (2) have metamorphosed rocks which were already cleaved, cut, and re-duplicated by the thrust-faulting at the time of their intrusion. The Glacial and post-Glacial accumulations are also described in outline.—E. S. Cobbold: Some small trilobites from the Cambrian rocks of Comley (Shropshire). Most of the trilobites were obtained during the progress of the excavations referred to in the report of the Geological Excavations Committee of the British Association, read at the Dublin meeting, 1908. The specimens were derived from the Olenellus Limestone of Comley, and from the Grey Limestones which intervene between that horizon and the Conglomeratic Grit, yielding a Paradoxides fauna.—J. B. Scrivenor: The rocks of Pulau Ubin and Pulau Nanas (Singapore). Pulau Ubin and Pulau Nanas are islands set in the eastern entrance to the Straits of Johore, and consist of igneous rocks of considerable interest. Pulau Ubin is composed mainly of hornblende-granite, but a pyroxene-bearing microgranite is found also, while the hornblende-granite is cut by rhombic-pyroxene bearing veins and also contains angular masses of rock resembling the veins. Pulau Nanas consists of dacite-tuffs and dacite, which are referred to the Pahang volcanic series, of Carboniferous or Permo-Carboniferous age. The tuffs and lavas have been altered by the adjacent granite of Pulau Ubin, and contain much secondary biotite and hornblende; their most remarkable feature is the presence of fragments of altered granite. The mutual relations of the different rocks are described. The normal granite of Pulau Ubin is hornblende-granite, the age of which is certainly post-Triassic and pre-Eocene, perhaps post-Inferior-Oolite and pre-Cretaceous. Veins of quartz-norite and masses of quartz-biotite-gabbro, and veins and masses of a fine-grained rock which may be described as enstatite-spessartite, are found in the normal granite of Pulau Ubin. A pyroxene-micro-granite and porphyry on Pulau Ubin, and a rock at Changi, having the mineral constitution of an amphibole-vogesite, are described. The dacite-tuffs of Pulau Nanas contain fragments of granite which must be of pre-Carboniferous age, and are referable to the granite of Amboyna. The fragments of granite, and perhaps certain pebbles of schorl-rock, are the only evidence found as yet in the Malay Peninsula of pre-Carboniferous rocks.—J. B. Scrivenor: The tourmaline-corundum rocks of Kinta (Federated Malay States). Overlying the limestone on the west side of the Kinta Valley is a thin cap of schists, with which are found certain rocks, the two chief constituents of which are tourmaline and corundum. They are often carbonaceous, and, in the many variations found, white mica, brown mica, pleonaste, rutile, and metallic sulphides occur. The tourmaline-corundum rocks of Kinta consist of varying amounts of tourmaline, corundum, carbon, white mica, spinel, and other minerals. They contain cavities about 6 millimetres in greatest width, generally bordered by a layer of corundum grains, with tourmaline grains on the inside of this border. Sometimes solid bodies similar in size and shape to the cavities occur. Smaller bodies occur, sometimes, but not always, accompanied by the larger cavities and bodies. They consist of tourmaline, of corundum, and of tourmaline and corundum. When both minerals are present the corundum forms a shell to a nucleus of tourmaline. The tourmaline-corundum rocks are associated with other rocks, which lead to the conclusion that the structures described are the result of replacement of the materials of preexisting bodies at the time of extensive granitic intrusions. They also are associated with rocks which point to the original beds having been laid down under conditions similar to those that obtained when the Pahang chert series was deposited. As tourmaline-bearing partings in the limestone at Changkat Pari constitute a case of selective metamorphism, so it is thought that the tourmaline-corundum rocks mark a process of intense metamorphism in beds associated with schists. These beds were probably chert and silicified limestone, both being in many cases carbonaceous. The larger cavities and bodies mentioned are believed to be the result of replacement of oolitic grains. The smaller bodies may be, in part, the result of replacement of the materials forming casts of radiolarian structures.
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Societies and Academies . Nature 82, 208–210 (1909). https://doi.org/10.1038/082208a0
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DOI: https://doi.org/10.1038/082208a0