﻿1851.] LOGAN— GEOLOGY OF SINGAPORE, 



327 



and curves seen in horizontal sections are generally portions of a con- 

 nected system, of which the external laminae are rectilinear, or nearly 

 so. The principal rectilinear planes of division approximate in their 

 general direction to N.E., S.W., or at right angles to the axis of ele- 

 vation of the region. 



Transverse and approximately horizontal planes of division exist, 

 so that the rock tends everywhere to break into spherical, cuhoidal, 

 and tabular masses. Its composition in many places varies with the 

 structure ; and the unequal wasting thereby occasioned has given rise 

 to very singular external forms in the exposed masses. The most 

 common are cuboidal rocks, from a few yards to twenty and upwards 

 in diameter ; having their sides traversed by concave vertical grooves, 

 varying in depth from a few inches to about six feet ; the sides of the 

 larger concavities being often grooved in their turn. These grooves 

 generally extend from the base to the summit of the rock, a height 

 in some cases of about 40 feet, although generally much less. On 

 the upper surfaces, and on the sides where they are inclined, a suc- 

 cession of hemispherical and pear-shaped cavities are frequently seen ; 

 and, where they are wanting, less regular furrows, corresponding with 

 divisional planes, generally supply their place*. 



The true explanation of all these phenomena I believe to be that 

 which I offered towards the conclusion of the paper mentioned in the 

 preceding note. It is as follows ; — " The blocks, protruding from 

 the hills or ranged along the shores of Pulo Ubin, are more solid and 

 less decomposable masses and nuclei, of which the forms, with the 

 directions of their sides and axes, have, in almost every instance, been 

 determined by structural planes, and which remain after the surround- 

 ing rocks have been disintegrated and washed away. "With respect 

 to the latter, it is obvious that while the island has been extending 

 by the growth of alluvium in its bays, its more open coast has been 

 slowly retreating, so that what was once a part of the solid land is 

 now a band on its border washed by the sea, but still exhibiting 

 numerous rocky remnants. The larger masses still evidently occupy 

 their original positions. Frequently their seaward face is curved. 

 Sometimes another mass stands behind, merely separated from that 

 in front by a chasm whose sides are parallel. With respect to the 

 decomposition of the rocks on the hills, the soil is entirely derived 

 from this source, with the exception of a very slight superficial mix- 

 ture of vegetable matter, which in many places is absent. In general, 

 however, the blocks that remain are decomposing with exceeding 

 slowness. One exception I noticed in the N.W.-S.E. side of a block 

 about 20 feet in height. The lamina? being inclined inwards, in dis- 

 rupting by their own weight fall some feet in front of the base, where 

 a long mound of earth has consequently accumulated. I have now 

 only to revert to the grooves. The circumstances attending them 

 which any hypothesis of their origin must explain are these : their 

 general prevalence ; the existence, however, of exposed rocks devoid 



* I have described the rocks of Pulo Ubin in detail in a paper communicated 

 to the Batavian " Genootschap van Kunsten en Wetenschappen," in 1847, and 

 published in the 22nd vol. of their Transactions. 



