Granitic Terrains

Granite underlies substantial areas of Southeast Asia. It forms the core of many of the major uplands. Yet exposures are scarce. High rainfall, consistently high temperatures, and the naturally abundant vegetation have together caused the granite to be deeply weathered. Most of the land surface is underlain by a more or less thick mantle of weathered rock or regolith. Only where the regolith has been removed by natural agencies, for example on some hill crests and steep midslopes, in river channels, and in coastal areas, is the bedrock naturally exposed, though road cuttings, quarries, and other artificial excavations provide excellent sections. Anthropogenically induced and accelerated soil erosion have also revealed bedrock morphology in places. The granitic terrains consist essentially of high ridges rising abruptly from the valley floors or adjacent plains. In detail, slopes, river channels, and rocky coasts strewn with granite blocks and boulders are characteristic of the region, and the nature of granite weathering has also influenced the character of the sediment load transported to rivers and coasts. Though the granites of Southeast Asia are well documented geologically and as sources of tin and other minerals, there are few modern accounts of their geomorphological aspects. Early travellers like Logan (1848) made astute observations relevant to the development of granitic forms, and the officers of the geological surveys of Malaya and, later, of Malaysia have, taking their lead from the first director onwards, noted salient features of the granitic terrains they mapped. These observations and interpretations, taken together with the few specifically geomorphological studies of particular features, and analyses of granitic landforms in other countries, permit the granitic terrains of Southeast Asia to be placed in context. Granitic rocks are widely distributed in Southeast Asia, particularly in the mainland states (Hutchison 1989). Those of the Malay Peninsula were emplaced at various depths: shallow epizonal, deep catazonal, but mostly mesozonal emplacement at 5–11 km depth. In plan, granites are widely distributed (Gobbett and Tjia 1973; Chinese Geosciences Research Institute 1975; UNESCO 1980). In the Malay Peninsula, granites occupy the cores of major regional anticlines, and many plutons are exposed in the breached crests of such structures.

Lithospheric roots beneath western Laurentia: the geochemical signal in mantle garnets

This study presents major and trace element data for 243 mantle garnet xenocrysts from six kimberlites in parts of western North America. The geochemical data for the garnet xenocrysts are used to infer the composition, thickness, and tectonothermal affinity of the mantle lithosphere beneath western Laurentia at the time of kimberlite eruption. The garnets record temperatures between 800 and 1450°C using Ni-in-garnet thermometry and represent mainly lherzolitic mantle lithosphere sampled over an interval from about 110–260 km depth. Garnets with sinuous rare-earth element patterns, high Sr, and high Sc/V occur mainly at shallow depths and occur almost exclusively in kimberlites interpreted to have sampled Archean mantle lithosphere beneath the Wyoming Province in Laurentia, and are notably absent in garnets from kimberlites erupting through the Proterozoic Yavapai Mazatzal and Trans-Hudson provinces. The similarities in depths of equilibration, but differing geochemical patterns in garnets from the Cross kimberlite (southeastern British Columbia) compared to kimberlites in the Wyoming Province argue for post-Archean replacement and (or) modification of mantle beneath the Archean Hearne Province. Convective removal of mantle lithosphere beneath the Archean Hearne Province in a "tectonic vise" during the Proterozoic terminal collisions that formed Laurentia either did not occur, or was followed by replacement of thick mantle lithosphere that was sampled by kimberlite in the Triassic, and is still observed there seismically today.

Influence of Propazine and Chlorthal Dimethyl on Mycorrhizal Development in Pinus radiata Seedlings

In an investigation into the side effects of herbicides on mycorrhizal formation in Pinus radiata nurseries, propazine and chlorthal dimethyl were added at concentrations that approximated to normal (2.20 and 1.88 g a.I-1L) and half-normal (1.10 and 0.94 g a.i.-1L) field application rates respectively to P. Radiata seedlings growing in terra cotta pots. A humus-rich, surface soil from a 40-year-old P. radiata plantation was used as mycorrhizal inoculum. Root length, mycorrhizal numbers and type, and seedling growth were assessed and a histological examination of bulk mycorrhizal samples was carried out. Propazine applied at both dosage rates and chlorthal dimethyl at the higher rate significantly sup- pressed root growth during the first 3 months and mycorrhizal formation during both the early and later stages of growth. However, neither herbicide affected shoot growth after 3 months and both increased shoot growth at the end of 6 months. Two mycorrhizal types were identified, a white-coloured, heavily dichotomised ectomycorrhiza with a thick mantle and a reddish-coloured, weakly dichotomous ectendomycorrhiza. The roots of chlorthal dimethyl treated seedlings strongly suppressed intracellular penetration in the ectendomycorrhiza, producing an unusual association with only a Hartig net, which was similar to that seen in senescing associations and some long roots of conifers. The ratio of white to red mycorrhizal numbers was highest in the untreated controls and was reduced by both herbicides. Both herbicides appeared to effect mycorrhizal formation through their influence on root morpho- genesis and host physiology.

Comparison of moraines formed by surging and normal glaciers

A Neoglacial moraine of Bighorn Glacier, St. Elias Mountains, Yukon, probably formed by a surging glacier, and a Neoglacial normal moraine of nearby Grizzly Glacier were studied. Surficial deposits and erosional features were mapped, Fabric analyses were done in till at five sites in each moraine. Samples were subjected to laboratory lithologic and texture analyses.The Bighorn (surge) moraine is a thin, discontinuous, irregular mantle, mostly till, but including ice-contact stratified material. Lateral moraines show subdued ridges marking the upper limit of glaciation. Ground moraine of the Grizzly (normal) glacier is an irregular, continuous, thick mantle, mostly till and. ice-contact stratified material. Upper ice limits are marked by prominent ridges. Rocks of similar lithologies within the till are concentrated in bands parallel to glacier flow.Stones in till of the Grizzly moraine have strong preferred orientation roughly in the direction of glacier flow. In the Bighorn moraine, rocks in till are weakly oriented. Where orientation is apparent, it is not necessarily in the flow direction. For the size fraction finer than 2 mm, Bighorn till is coarser than Grizzly till although the bedrock terrain is similar.These differences are useful in distinguishing moraines of normal and surging glaciers.

The use of satellite signals to investigate the polar ionosphere

The ionosphere is a thick mantle of partly ionized gas surrounding the earth at heights between 80 and 800 km. It was discovered about fifty years ago, and during the last decade has been studied intensively at observatories all over the world. These studies consist primarily of observing the characteristics of radio waves reflected from the ionosphere. Since waves of different frequencies are reflected at different heights, it is possible to determine how the electron density varies with height, up to the height of maximum density at about 300 km. Radio waves which pass this height, however, continue into space and are not reflected. Consequently very little direct information can be obtained from the ground about the characteristics and behaviour of the ionized region above 300 km.

V.—Another Section of Keuper Marls at Great Crosby, Lancashire

In 1884 I described in the GeologicalMagazine a section of Keuper Marls exposed by the excavation of the Boulder-clay at Moorhey, Great Crosby. This was our first knowledge of their existence in the neighbourhood, the whole area being covered with a thick mantle of Boulder-clay excepting where the Lower Keuper Sandstone comes to the surface in the villages of Great and Little Crosby.

I.—The Cause Of The Glacial Period, With Reference To The British Isles

It is a fact universally accepted that, within a period comparatively recent, extensive districts in North America and in Europe, now fruitful and luxuriant, were covered with a thick mantle of snow and ice; and their valleys were filled with glaciers, which extended into the sea, and, breaking off at their extremities, floated away as icebergs.