Philip Lake— Form of Mountain Chains. 305 



IV. — The Circular Form of Mountain Chains. 

 By Philip Lake, M.A.,F.G.S. 



I AM a stranger in the field of speculation, and am quite un- 

 acquainted 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 sub- 

 merged) 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 thrust- 

 plane. 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. On 

 the scale of an ordinary globe the earth is sensibly a sphere, and 

 therefore any deviation which is visible on such a globe must 

 be produced by a deviation of the surface of movement from 

 a true plane. 



Further, in the case of a circular mountain range it is possible 

 from the form of the arc to determine the dip of the basal thrust- 

 plane ; for it is easy to show that the angle which a plane makes 

 (at its outcrop) with the surface of the sphere is equal to the 

 angular distance, measured on the surface of the sphere, between 

 the centre and the circumference of the circle formed by the outcrop 

 of the plane.^ 



Thus, for example, the centre of the Himalayan arc is placed by 

 Professor Sollas in lat. 42° N. and long. 90° E., and the angular 

 distance from this point to the arc is 14:°. This, then, is the angle 



1 Quart. Journ. Geol. Soc, 1903, p. 180. 



2 The very beautiful sections of the oiiter 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. 



^ 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. 



DECADE IV. — VOL. X. NO. Til. 20 



