In a communication to the GeologicalMagazine, published in July, 1936, I gave a brief description of the rocks that build the Kedong scarp. I propose now to amplify that description. It is not always realized that the geological structure of the Rift Valley has been deduced almost wholly from physiographic evidence, and that proof of the alleged displacements must depend upon exact petrographic study of the rocks involved in the structures.

Later Cambrian and earliest Ordovician trilobites and brachiopods spanning eight horizons from five localities within the Sông Mã, Hàm Rồng and Đông Sơn formations of the Thanh Hóa province of Việt Nam, constrain the age and faunal affinities of rocks within the Sông Đà terrane, one of several suture/fault-bounded units situated between South China to the north and Indochina to the south. ‘Ghost-like’ preservation in dolomite coupled with tectonic deformation leaves many of the fossils poorly preserved, and poor exposure precludes collecting within continuously exposed stratigraphic successions. Cambrian carbonate facies pass conformably into Lower Ordovician carbonate-rich strata that also include minor siliciclastic facies, and the recovered fauna spans several uppermost Cambrian and Lower Ordovician biozones. The fauna is of equatorial Gondwanan affinity, and comparable to that from South China, North China, Sibumasu and Australia. A new species of Miaolingian ‘ptychopariid’ trilobite, Kaotaia xuanensis, is described. Detrital zircon samples from Cambrian–Ordovician rocks of the North Việt Nam and Sông Đà terranes, and from Palaeozoic samples from the Trường Sơn sector of Indochina immediately to the south, contain a predominance of ages spanning the Neoproterozoic period and have a typical equatorial Gondwanan signature. We associate the Cambrian and Tremadocian of the Sông Đà terrane with areas immediately to the north of it, including the North Việt Nam terrane and the southern parts of Yunnan and Guangxi provinces of China.

The series of fossil plants from the Tertiary strata of New South Wales and Tasmania, to which these remarks relate, were sent to Mr. R. Etheridge, jun., at the British Museum, for examination partly by Prof. Liversidge, of Sydney University, and by Mr. C. S. Wilkinson, F.G.S., Government Geologist for New South Wales, whilst the remainder already formed a portion of the National Collection.

The Jurassic magmatic record in the southern Colombian (Northern Andes) includes numerous subduction-related I-type calc-alkaline granitoids with diverse structures and textures, formed in two main episodes at ∼195 to 165 Ma and ∼165 to 145 Ma. We provide new insights into the mineral chemistry, estimates of intensive parameters and petrogenetic processes of 12 plutonic occurrences in the region, grouped in 4 petrographic associations. Primary mineral assemblages include labradorite-to-oligoclase, alkali feldspars, ferroan enstatite, Mg-rich augite to ferroan-diopside, tschermakite to hastingsite and hornblende and Mg-rich annite; Fe-rich phlogopite and actinolite are post-magmatic phases. Amphibole chemistry indicates that the older (195–165 Ma) Jurassic bodies formed from relatively highly oxidized (fO2 values buffered at −0.1 ≤ NNO ≤ +1.4) hydrous (∼4 to 6 wt % H2O) magmas and their differentiation involves significant crustal assimilation and/or magma mixing, fractional crystallization and late-magmatic re-equilibration processes. In contrast, the younger (165–145 Ma) Jurassic intrusives, derived from subducted-modified mantle sources, record moderately lower oxidized hydrous conditions (fO2 values −0.7 to 0.8 ≤ NNO; ∼5 wt % H2O) with magma evolution mainly controlled by fractional crystallization and late-magmatic re-equilibration processes. Clinopyroxene-only, amphibole-only and amphibole-plagioclase thermobarometry estimations suggest that the Jurassic occurrences crystallized over variable temperature (647°C–1087°C) and pressure (0.7–6.3 kbar) conditions, corresponding to emplacement depths ranging from ∼15, ∼8 to 11, ∼5 to 7 and <4 km along the arc crustal column. The obtained data combined with time evolution allow the identification of exhumed and fragmented arc blocks in the Jurassic magmatic system and provide an essential link between the orogenic deformation event poorly constrained in the Northern Andes.

The Chahmileh Pb–Zn deposit, located northwest of the Central Iran Zone, is a sediment-hosted Pb–Zn deposit in the ‘Yazd-Anarak Metallogenic Belt’. It is hosted in Middle Triassic carbonate rocks and is mainly controlled by NW-trending faults. The main ore minerals are galena and sphalerite with minor chalcopyrite, pyrite, and quartz, dolomite, along with minor calcite and baryte as gangue minerals. Cerussite, hemimorphite, wulfenite, mimetite, smithsonite, malachite and iron oxy-hydroxides are the main non-sulphide ore minerals. The main styles of mineralization are vein-veinlet, breccia, disseminated and replacement in association with silicification and dolomitization. Microthermometry of fluid inclusions in dolomite and quartz indicates that the ore precipitated from a warm to hot basin-derived saline fluid. Dolomite samples have δ13CVPDB and δ18OVSMOW values of −0.99 to +1.99‰ and +20.74 to +25.48‰, respectively, and are plotted in the marine carbonate rocks field. These isotopic values suggest that CO2 in the hydrothermal fluids mainly originated from marine carbonate rock. The δ34S values range from +6.3 to +8.2‰ for galena, +5.9 to +6.2‰ for sphalerite, +1.4 to +3.4‰ for chalcopyrite and +15.0 to +17.4‰ for bayite are compatible with a predominant thermochemical sulphate reduction process, and with sulphur sourced from Triassic seawater. Galena samples have a homogeneous Pb isotopic composition that is indicative of a continental crustal reservoir as the main source of lead and probably for the other ore metals. Based on geology, mineralogy, texture and fluid characteristics, the Chahmileh deposit is classified as a carbonate-hosted Mississippi Valley-type deposit.

Few amongst the many remains of Fossil Fishes hitherto discovered have excited more interest than the remarkable fossil Ray, found about forty years ago by Miss Anning in the Lower Lias at Lyme Regis.

The use of the term mugearite as a synonym for oligoclase-basalt is discussed. It is shown that the two rock-types, though often agreeing closely in chemical composition, may differ appreciably in their fabric and mineralogy.

Quenstedt described three distinct varieties of Ammonites Davœi, with nine illustrations. They all belonged to the 7 zone of Lower Lias:—

1. Ammonites Davœi, whose tubercles are somewhat clavate and with unsymmetrical prorsiradiate ribs.

2. A. Davœi enodis is continental. It is a small specimen without tubercles, with fine symmetrical and prorsiradiate ribs.

3. A. Davœi nodosissimus is also continental. This variety shows nine large bullate tubercles on each whorl, with close, fine, and symmetrically disposed flexiradiate ribs.

Having frequently visited Upware during the past fourteen months, for the purpose of collecting from the Lower Greensand, in which I have been very successful, having obtained for this Museum a beautiful and choice collection, I have had every opportunity of observing the progress of the excavations, and of noting the relative position of the beds.

A traverse of a noritic gabbro sill (c. 1,000 feet thick) intruding the Roraima Formation showed no cryptic layering, but small-scale fractionation throughout the sill is indicated by the strongly-zoned plagioclase (An80→40), and orthopyroxene (En79→54) with outer rims of inverted pigeonite. Olivine (Fo80) is only present in the chilled margin.

New evidence on the age of the granites underlying the Roraima Formation is given by the discovery of post-Roraima felsite dykes, with high-quartz phenocrysts.

A New and interesting species of Trilobite having lately been obtained by Professor A. Liversidge, F.C.S., F.G.S., of the University of Sydney, in the Silurian rocks of Bombala, New South Wales, and forwarded to my colleague, Mr. R. Etheridge, jun., F.G.S., with a series of other Palæozoic fossils, from Australia, it has been obligingly placed in my hands for description.

Bedding mullions have been produced by compression parallel to bedding in the hinge region of a large recumbent fold. Geometrically, the mullions are a combination of parasitic folding, with characteristic gadrooned profile, and cleavage planes. The cleavage is not an axial plane structure, but where fully developed produces a perfect slate fabric in rocks of appropriate lithology. The mullions are found on both sides of a bed of inferior slate sandwiched between two beds of good roofing slate. Boudinage structure results where the mullions are exactly back to back, and the planes of separation provided by the cleavage are infilled with coarse calcite. The slates are interbedded and interlaminated with limestone.

Such boudins are unusual in that they occupy the hinge region of a fold and result from compression parallel, rather than at right angles, to the bedding.

The Merjelen or Maerjalen Lake, situated at an altitude of 2367 metres (7750 ft.) above sea-level at the western flank of the Great Aletsch glacier, belongs to the class of glacier lakes which are found in depressions or valleys barred by glaciers whose direction of flow is more or less at right angles to the same. Like the majority of glacier lakes, it has the shape of an irregular triangle, the length being about 1.5 kilometre (nearly a mile), its greatest width 0.5 kilometre (550 yards), and its mean depth 28 metres (92 ft.). Hence its superficial area amounts to 375,000 square metres (438,000 square yards), and its volume at high-water level to about 10 million cubic metres or tons.

Columnar-jointed tuffs (bentonites) are located below the Lower Carboniferous Tideswell Dale Sill, Derbyshire, in an abandoned quarry. There are three zones of prismatic joints, columnar joints (∼60 cm thick and mean column width of 4.1 cm) and massive, conchoidally fractured rock sequentially from the dolerite contact downwards. The rocks are very fine-grained (1–10 µm) and consist mainly of sanidine, interlayered illite/smectite, and hematite, with minor quartz, apatite, montmorillonite, anatase and detrital minerals. High K2O and Fe2O3, low Na2O and low MgO are interpreted as due to alteration of felsic volcanic ash. The bentonite was contact-metamorphosed by the sill to temperatures of c. 300 °C. The progression of fracture geometries, thermal considerations and application of theories of column formation indicate that columns formed downwards away from the sill, due to prograde contact metamorphism of the originally clay-rich bentonite. The formation of columnar fracture networks by shrinkage due to igneous heating may have implications for the effectiveness of bentonite as a barrier for disposal of high-level radioactive waste. Regional dimensions include documenting widespread felsic explosive volcanism and inferring a Triassic oxidation event from palaeomagnetic data.

An example of the development of axial plane schistosity close to a buckled competent band is described. There is a difference in the degree of development of the schistosity on the convex and the concave sides of the fold arch. This is explained as due to variation in contact strain. A similar pattern has been produced experimentally and it is concluded that the axial plane schistosity in the example under consideration is a mechanical phenomenon and has developed perpendicular to the direction of maximum compressive strain.