BY E. M. JOHNSTON, F.L.S. 51 



in the cooling or the radiation of heat into space there were 

 areas of greatest and least contraction. This difference in 

 cooling and the resulting level of the surface must have been 

 owing to some difference of quality or condition in the 

 material." One quality has been brought to light by pendulum 

 experiments in India, proving that gravity is greater at the 

 coast stations than at the continental stations, and greater at 

 the island stations than at the coast stations. In harmony 

 with these observations are the opinions advanced by 

 Archdeacon Pratt (The Figure of the Earth, 1860 ; on the 

 Constitution of the Solid Crust of the Earth, Nature, 1871) 

 who first attributed the origin of oceanic depressions 

 and continents, and also of mountain chains, to unequal 

 contraction in a cooling globe, but in the last refers the 

 formation of mountains to lateral pressure, and there con- 

 cludes " That the crust beneath the ocean is of greater density 

 than the average portions of the surface — that is, where the 

 contraction was greatest the density of the rich material 

 below is greatest and proportionately so.'' Mr. George H. 

 Darwin, in investigating the bodily tides of viscous spheroids 

 (Phil. Trans., 1879), among other important matter also points 

 out that under the combined influence of rotation andthe moon's 

 attraction the polar regions tend to outstrip the equator, and 

 to acquire a consequent and slow screwing motion from east to 

 west ; and Dr. Geikie, who has certain objections to offer, 

 admits in respect of Darwin's theory, that it is conceivable in 

 earlier conditions of the planet, that this screwing motion 

 mav have had some influence in determining the surface 

 features of the planet. In a body not perfectly homogeneous 

 it might originate wrinkles at the surface running perpendicular 

 to the direction of greatest pressure. "According to this 

 theory the highest elevations should be equatorial, and should 

 have a general north aud south beud, while in the northern 

 hemisphere the main direction of the masses of land should 

 bend round towards the north-east, and in the opposite 

 hemisphere towards the south-east." While there are many 

 difficulties standing in the way of the full acceptance of Mr. 

 Darwin's theory, Dr. Geikie still thinks " It is well worth 

 consideration whether the cause suggested by Mr. Darwin 

 may not have given their initial trend to the masses of land, so 

 that any subsequent wrinkling of the terrestrial surface due to 

 any other cause ivould be apt to take place along the original 

 lines." I might also add that to such influence in some 

 measure might be still ascribed those remarkable lines of 

 weakness in both hemispheres, which mark the course of 

 volcanic action, and which, in a large measure, may determine 

 the curves of certain coastal mountain chains. Apart, however, 

 from the universality or antiquity of alleged causes advanced 

 as the principal factors in mountain making (I here use the 



