5-28 On the Local and Relative Geology of Singapore, [June, 



3. — Decomposition of Rocks in Situ. 

 This, which is the hypothesis that next most naturally arises, would 

 embrace many of the facts that are inconsistent with the sedimentary 

 and diluvial suppositions, such as the local character of the rock frag- 

 ments. The outcrops of the strata, which are generally highly inclined, 

 would under meteoric influence, down to a certain line of depth which 

 would descend with the denudation of the surface, suffer different changes 

 according to the nature of the rock. The harder sandstones and shales 

 would, split and break down into irregular fragments. The softer 

 sandstones, clays and shales, — and of the latter especially the finely 

 laminated beds, — would, under the combined chemical and mechani- 

 cal influences of the air, rain, rapid transitions of temperature, &c, 

 lose their distinctive original characters and gradually become uniform 

 masses of sandy or clayey soils. Every heavy fall of rain would wash 

 away the more superficial particles. According to the declination of 

 the sides of the hills, fragments of rock of different sizes would be 

 carried down by the pressure of water-moved soil and gravelly frag- 

 ments. Where the hills were steep, larger blocks, from the gradual 

 loosening of their beds, would descend to lower levels by their own 

 gravity assisted by similar pressure from above. The summits and 

 ridges of the hills would be most exposed to the action of sun and rain, 

 but generally least so to the denuding power of gravity. Where the soil 

 was loose sand, or where there were narrow summits, the process of 

 denudation would be more active than elsewhere. The soil as it was 

 formed would disappear, and only fragments of rock be left where the 

 latter was of a nature to yield with difficulty, slowly and superficially 

 to decomposition. Where the fragments pulverized more quickly, 

 some soil would generally be found, always drawing additions from the 

 rocks, but always a prey to the rains. 



These considerations certainly explain the present appearance of many 

 of the hills, and in every locality phenomena occur evidently due to 

 the forces of which I have been writing. Ridges and summits are 

 often found consisting almost entirely of rock fragments, and it might 

 seem that these forces alone would be adequate causes for their occur- 

 rence. But on hills with extensive flattish summits, beds of fragments, 

 sometimes large, — sometimes of all sizes mixed — sometimes uniformly 

 small and gravel-like, lying under or in the soil at various depths, from 



