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Published online by Cambridge University Press: 01 May 2009
I am a stranger in the field of speculation, and am quite unacquainted with the intricacies of its authorized boundaries. It is therefore with some hesitation, lest I should tread upon forbidden ground, that I venture to offer a suggestion on one point in Professor Sollas's paper on “The Figure of the Earth.”
It has long been observed that mountain ranges and chains of islands (which, indeed, are only mountain ranges partially submerged) are generally curvilinear in form, but Professor Sollas is, I believe, the first to show clearly that the curve often coincides almost exactly with an arc of a circle. Such a mountain chain is frequently defined along its convex margin by a great reversed fault over which the mountain mass has slid forward; and in these cases, at least, we may safely adopt Suess's conception, and look upon the chain as the crumpled edge of a ‘scale’ of the earth's crust which has been pushed forward over the part in front of it. The surface along which the movement has taken place is called a thrustplane. If this surface really is a plane, then the edge of the ‘scale’, that is the mountain chain itself, must necessarily be circular in form; for if any plane cuts a sphere, in any position whatever, the outcrop of the plane on the surface of the sphere will always be a circle. There can be no deviation from the circular form unless the ‘sphere’ is not truly spherical, or the ‘thrust-plane’ is not a true plane.
page 305 note 1 Quart. Journ. Geol. Soc., 1903, p. 180.
page 305 note 2 The very beautiful sections of the outer Himalayas given by Mr. Middlemiss (Mem. Geol. Surv. India, vol. xxiv, pt. 2) illustrate the process in operation, and in the North-West Highlands of Scotland we have the actual base of such a mountain chain exposed to view.
page 305 note 3 It should be noticed that a spherical surface also, if it cuts a sphere at all, will necessarily have a circular outcrop. No other form of surface, I believe, has a similar outcrop, excepting only in certain definite positions. A cylindrical surface, for example, will crop out in the form of a circle, if its axis coincides with a diameter of the sphere, but not otherwise. If the thrust-plane be a portion of a spherical surface, it is impossible to determine its dip from an inspection of its outcrop alone.