Mafic dykes from North America and India are used to highlight certain structural features that may provide further insight into the origin and geodynamic significance of dyke swarms, particularly those of Precambrian age. Structural aspects of interest within any one swarm include regional variations in dyke attitude, perference in direction of dyke branching, radiating dyke patterns, and orthogonality of dyke trends with the structural grain of enclosing host rocks. Geological observations suggest that sub-horizontal magma flow is a common feature of dyke intrusion and thus changes in petrology, geochemistry and the orientation of flow-induced fabric along the length of a swarm might be expected which would have important bearing on problems concerning location of magma sources and the process of dyke injection.
Paleosols, particularly buried paleosols, have proved valuable stratigraphie markers in many Quaternary studies. They are most commonly found buried by loess in central Europe and the United States, and by volcanic ash in the Pacific margins. Within their morphologies are recorded evidence of past climates and vegetations. The understanding of paleosols has paralleled our comprehension of contemporary soils and hence the methods of investigation of paleosols are those that have found common usage in the study of the soils of today. The recognition of paleosols is thus largely based on the identification of characteristics known in contemporary soils.
The usefulness of paleosols as stratigraphic indicators is in part due to their extensive and recognisable occurrence over large areas and because they often contain material suitable for absolute dating by the 14C method. Wood, charcoal, peat and soil organic matter have all been employed in the dating of paleosols. 14C dating of secondary carbonates or oxalates and of plant opal phytoliths, whilst more difficult than the dating of wood, etc., are also possibilities for the Quaternary paleopedologist. Relative dating techniques, particularly changes in the composition of organic matter after burial, have also proved useful in some studies.
The reconstruction of paleo environments from paleosol morphology presents many problems and conclusions must be derived from as many lines of evidence as possible. Pollen, opal phytoliths, faunal remains, micro morphology and mineralogical composition should all be investigated and the results integrated to provide a consistent picture of past climates and vegetations. A lack of understanding of pedology can often lead to erroneous conclusions.
Simple conductive geotherm calculations provide a useful way of illustrating the thermal constraints on metamorphism in teaching senior undergraduate classes. In this article a variety of one-dimensional problems is examined to show the effects of variation in conductivity, heat production and mantle contribution, and also to show the time needed for a rock column to reequilibrate thermally after erosion.