﻿1851.] 



LOGAN — GEOLOGY OF SINGAPORE. 



337 



their present remnant by denudation, they would not have been 

 broken up in the manner in which we see them to be, but in a mode 

 resembling that in which the great aqueous formations of Europe and 

 America are arranged around or on the flanks of plutonic mountains, 

 and in which the strata of the ancient land may perhaps yet exist in 

 some parts of the flanks of the great intumescence or submarine ele- 

 vation of the Malayan Archipelago where it rises from the bottom of 

 the Indian Ocean. 



1 2. The prevalent layers and heaps of volcanic-like gravel were 

 not ejected. They are merely the accumulated debris of walls and 

 veins arising from the decomposition and washing away of the softer 

 parts of the containing rock, and remaining by virtue of their ferru- 

 ginous and jaspideous composition, which resists atmospherical action. 



I have found all the forms of the gravel in the places where they 

 were generated, — that is, in the walls traversing the solid or partially 

 disintegrated rocks. 



13. The iron was not injected in a fluid state into the strata, as 

 lava is into fissures, but was either imbibed, or, as is most probable, 

 conducted by vapours, gases, or electric currents. The ferruginous 

 an'd quartzo-ferruginous rocks, including laterite, wherever minutely 

 examined, prove to be the original rock of the situs metamorphosed, 

 and not a foreign rock injected from beneath ; and there are evidences 

 in all classes of the rocks affected, that the iron, although ascending 

 in abundance wherever the rocks were fissured or weakly coherent, 

 was thence transfused without its exerting any mechanical force, and 

 without the slightest disturbance of the previous arrangement of the 

 particles of the rock*. 



14. It is probable that the plutonic rock was not in a molten state 

 up to its contact with the strata, because if it had been, the fissures, 

 which must have been caused by the irregular force exerted on the 

 strata and their frequent disruption, would in that case have given 

 rise to true dykes and veins, i. e. those formed by fluid plutonic or 

 volcanic rock being pressed up into rents. 



15. The numerous ferruginous dykes which ramify in the plutonic 

 rocks were contemporaneous, because if we suppose them to have 

 been true dykes produced subsequent to the intumescence and con- 

 solidation of these rocks, the enormous force, necessary for rending 

 and cracking them in so many directions, would have extended the 

 fissures into the superincumbent strata, and the injected dykes would 



* The entrance of the iron was, however, in many cases simultaneous with me- 

 chanical strainings and ruptures of the rocks, and, when rising abundantly into the 

 main walls and forced thence into the lateral ones, must have exerted considerable 

 pressure. The mechanical straining and the exhalations were both effects of the 

 same cause, the plutonic intumescence. The only case which I have observed of 

 a direct connection between the matter of the walls and a mechanical alteration 

 in the adjacent rock is seen when quartz is largely developed in lumps in the 

 quartzo-ferruginous dykes. The adjacent unaltered layers are pressed together 

 and bent so as to make room for the quartz. Even here I do not think this is 

 attributable to a large accession of new matter, but mainly to the expansion of 

 bulk attending the conversion of the siliceous particles of the original rock into 

 crystalline quartz. 



VOL. VII. — PART I. 2 A 



