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  • 1955-1959  (33)
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  • 1
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iThe 5 per cent and 1 per cent significance points are given for a test of randomness of unit vectors in three dimensions. The test has been designed for use in the analysis of palaeomagnetic data.
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iGravity data are used to investigate the geological structure of an area of about 160000 square miles in the light of the crustal warping hypothesis. The region is in N.W. Pakistan and India, and includes the whole of the alluvial plains of Sind and Punjab, the Salt Range and the Potwar Plateau.A new gravity anomaly is introduced and used by an original method, which leads automatically to the detailed contouring of the basement rock below the region, the basement being assumed to be the upper surface of the Earth's crust. The standard crustal section employed is a two-layer crust with a total thickness of thirty kilometres but a table permits direct comparison with other sections of a six-layer crust, two of which have increased thickness.The basement contours show a ridge about 300 miles long separating the Indus Basin from the Lahore Basin. It is hidden by alluvium except for a few outcrops near its northern end. This ridge has apparently suffered sub-aerial erosion under typical S.W. monsoon conditions, extending in places to a depth of over 3000 feet below sea level. This modification of the basement requires a revision of the contours over the ridge, and two contoured charts show firstly the simple crustal upwarp underlying the ridge and secondly the eroded surface of the basement. A deep valley with its bottom far below sea level cuts through the ridge connecting the Indus and Lahore Basins. In Sind a similar valley leads from the direction of the sea to the Indus Basin, but here interpretation is uncertain.It is concluded that the hypothesis yields results giving depths to the basement of the right order in deeply downwarped areas, but in upwarped areas the possibility of erosion, or other concealing factors, leads to uncertainty of interpretation unless the area is wide enough to include a complete section of the upwarp.
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  • 3
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iTerrestrial heat flow has been measured in three Alpine railroad tunnels. The geothermal gradients were calculated from temperatures measured during the construction of the tunnels, and corrections for topographic irregularities were made. The thermal conductivity of 113 rock specimens from the vicinity of the tunnels was measured. The heat flow in the Gotthard tunnel was found to be 1.6 10-6 cal/cm2 sec, in the Simplon 2.2 10-6 cal/cm2 sec, and in the Loetschberg 1.9 10-6 cal/cm2 sec. Most of the flux at the surface can be attributed to radioactive decay in a thickened crust, but a non-uniform distribution of radioactive elements may be required to explain the relatively high heat flow in the Simplon and Loetschberg tunnels.
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  • 4
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: iA detailed study has been made of the remanent magnetization of five Pilansberg dykes, by means of measurements on oriented specimens of the rocks. Outcrops of the dykes prove to have suffered magnetic disturbance of their original thermo-remanent magnetization. Specimens from depths of a few thousand feet, taken in Witwatersrand gold mines, show highly consistent magnetisation of the basic parts of the dykes over considerable distances between sampling sites. The mean directions given by the five dykes agree well, and the mean direction from the five dykes gives a North-seeking magnetic pole with inclination +69.3 and azimuth N 24° E. Assuming thermo-remanent magnetisation by a geocentric dipole field, this places a North magnetic pole in latitude 71/2° N, longitude 421/ E at the time of intrusion of the dykes. The age of the dykes is uncertain, but is probably about 300 to 400 million years. Some of the specimens have been subjected to alternating magnetic fields in order to test the stability of their magnetisations. The basic specimens are found to be highly stable, being only slightly affected by fields of 100 to 300 oersteds. The possible implications of the results are discussed, in terms of hypotheses of polar wandering and continental drift.
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The free (420d) nutation is known to be heavily damped, the characteristic damping time being less than ten periods. The origin of this damping is unknown, but must be due to dissipative, non-rigid-body movements of the Earth. The view, which has been widely held, that this may arise from the relative motion between the liquid core of the Earth and the mantle is shown to be based on an error. When account is taken of its small moment of inertia, the core cannot be held responsible for this damping, nor can a limit be derived for its viscosity. This confines the origin of the damping to a non-elastic behaviour of the mantle.
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  • 6
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Summary. This paper describes the statistical techniques available to the experimenter in palaeomagnetic work. The theory of these methods is based on an assumed probability distribution of errors. It is shown that the mathematical requirements of this distribution are obeyed by the observations from rock samples which are known to possess a stable magnetization; observations on rocks with unstable magnetization however do not conform to it. A theoretical derivation is given for this probability distribution.The problem of estimating the mean direction of magnetization of a geological formation has in recent years become a matter of the greatest geophysical interest since it is from such estimates that the position of the pole of the Earth in past geological ages is determined. This problem is largely one of the judicious choice of samples and a procedure is suggested whereby such estimates may be achieved with the greatest sample economy.
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  • 7
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Summary. The error is investigated when the mean of two solutions of a non-linear second-order differential equation which gives the variation of density with depth in certain layers of the Earth is used as a solution.A solution (Model M) of the equation for layer E is given, which has a density of 9.385 g/m3 at the top of layer E.Identifying, at the top of layer E, the mean of two numerically estimated solutions with a similarly estimated solution, it is shown that the error is a maximum at the bottom of layer E, where it is of the order of 0.04 per cent.
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Summary. The range of possible density distributions in the mantle of the Earth has been examined assuming a chemically homogeneous core. A discussion of various Earth models with homogeneous cores shows that the range is relatively small in the upper part of the mantle. For a density near the surface between 2.8 and 4.0 g/cm3, the density at 1000 km is between 4.1 and 4.8 g/cm3, and at 2000 km is between 5.2 and 6.5 g/cm3.Graphs showing the distributions of density, gravity, pressure, and elastic parameters in two fairly extreme models are given. The first model has a density jump at the core boundary of 4.2 g/cm3 and only slight heterogeneity in D. The second has a continuous density distribution throughout the Earth and large heterogeneity in D.
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A deep depression moved rapidly NNE from the Welsh coast across northern England to the Firth of Forth on 1954 December 9. At the same time microseisms of very large amplitude were recorded at Aberdeen, the maximum effect occurring when the centre of low pressure was situated over land. If the disturbances were due to standing waves set up in the sea in accordance with the Longuet-Higgins theory, these waves must have arisen behind the low centre by reflection from the coast. There was no other low centre in the neighbourhood likely to produce the effects. So far as the nature of the microseisms is concerned, the records support the idea that they consist of a mixture of Rayleigh waves and Love waves, and that in the present case they approached Aberdeen from a direction approximately SSW of the station.
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The frequency equation for Stoneley-type waves with symmetric vibrations propagated along the interfaces between an internal stratum and two adjacent halfspaces, all perfectly elastic, homogeneous and isotropic, is obtained as the vanishing of a determinant of the fourth order. For large values of the frequency, the equation reduces to that of Stoneley waves at the interface between two halfspaces. Discussion of the sign of this determinant for suitable values of the unknown leads to the condition for the existence of such waves both for a low-velocity and a high-velocity internal stratum. The ranges of values which the ratios of the elastic constants of the stratum and either halfspace must have are obtained by numerical computation and the results are presented both in tabular form and graphically.Tt is found that: (i) Stoneley waves with symmetric vibrations, when they exist, have their phase velocity lying between the distortional and Rayleigh wave velocities of the lower velocity medium, (ii) If, as is usual, a velocity ratio less than unity is associated with a density ratio less than unity, then such waves cannot exist unless the smaller of the two distortional wave velocities is greater than the higher Rayleigh wave velocity, (iii) As the frequency of these waves or the thickness of the stratum is decreased, there is a cut-off value of either below which Stoneley waves cannot be propagated.
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