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Mountain and Island Arcs

Published online by Cambridge University Press:  01 May 2009

Philip Lake
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In a recent number of the Records of the Geological Survey of India, vol. lxii, p. 410, Dr. Fermor has done me the honour to draw attention to a paper of mine (Geological Magazine, 1903, p. 305) in which I came to the conclusion that the thrust-plane at the base of the Himalayas must have, at its outcrop, a dip of about 14°; and he points out that this estimate agrees with the observations of Mr. Middlemiss in Jammu Province. My estimate was based upon a simple geometrical deduction which proceeds directly and inevitably from the assumption that the Himalayas rest upon a plane thrust-surface.

Type
Original Articles
Information
Geological Magazine , Volume 68 , Issue 1 , January 1931 , pp. 34 - 39
Copyright
Copyright © Cambridge University Press 1931

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References

page 34 note 1 Rec. Geol. Surv. India, 1, 122.Google Scholar

page 34 note 2 Quart. Journ. Geol. Soc., lix, 1903, 180.Google Scholar

page 35 note 1 The following considerations may help those who are not familiar with the geometry of the sphere. If the plane of any circle upon a sphere passes through the centre of the sphere the circle is a “great circle”, otherwise it is a “small circle”. On an ordinary globe the meridians and equator are great circles, the parallels of latitude, except the equator, are small circles. The pole of a small circle is a point on the surface of the sphere which is equally distant from every point of the circle, and it bears precisely the same relation to the small circle that the geographical pole does to a parallel of latitude. In each case there are two opposite poles, but we need only consider the one nearest to the small circle in question. It will be evident to everyone who looks at an ordinary geographical globe that the plane of any parallel of latitude at right angles to the planes of the meridians, i.e. to the planes of all the great circles that pass through the geographical pole. In the same way the plane of any small circle is at right angles to the planes of all the great circles that pass through its pole. When two or more small circles have their poles upon the same great circle, the plane of each small circle is at right angles to the planes of all the great circles passing through its own pole; and therefore the planes of all the small circles are at right angles to the plane of the one great circle that passes through the poles of them all.

page 36 note 1 I took the line of the fault from Richthofen's tectonic sketch-map of Japan in Sitz. d. k. preuss. Akad. Wissensch., 1903, 894.Google Scholar

page 37 note 1 Sitz. d. k. preuss. Akad. d. Wissensch., 1902, pl. iii. The map is reproduced in the French edition of Suess's Das Antlitz der Erde, iii, 1401.Google Scholar

page 37 note 2 To avoid encumbering the text with figures which are only of use to those who wish to check my calculations I give here the latitudes and longitudes of the points selected in each arc.

page 38 note 1 See Proceedings, Kon. Akad. Wetensch. Amsterdam, xxxiii, 563–77. The R.G.S. paper is not yet published.Google Scholar

page 39 note 1 A point to which it may be well to draw attention at once is that while the front of the overthrust mass is convex towards the direction of overt-thrusting, the front of the underthrust mass will be concave towards the direction of underthrusting.