﻿1851.] 



LOGAN — GEOLOGY OF SINGAPORE. 



341 



veins, the presumption is that these are the same walls and veins by 

 which the metamorphism commenced. This is a simpler theory than 

 that which would require the process of plutonic assimilation, as it 

 advanced, to have first obliterated those originally formed, and then 

 created a new system so remarkably similar to the old one. In what 

 way could such systems of ferruginous walls and veins have been pro- 

 duced de novo in a plutonic mass thoroughly melted down into uni- 

 formity 1 If it were clearly demonstrated as a general theory that 

 plutonic rocks are congealed from a state of fluidity, it might, not- 

 withstanding these and other difficulties attending its application to 

 the rocks of the district *, require us to reject the simpler theory 

 which these rocks inculcate. But plutonic geology is in too imper- 

 fect a state to entitle us to assume any general theory as established, 

 and nothing but the assumption that granite is necessarily, every- 

 where, a product of complete igneous fusion, requires us to suppose 

 that two successive systems of ferruginous walls were produced, one 

 at the commencement, and another before the conclusion, of the plu- 

 tonic change. 



If we are satisfied to suppose that only one system was produced, 

 until some proof be adduced that it could not possibly have remained 

 unobliterated during the plutonic process, these walls and veins be- 

 come objects of the highest interest and importance, and all the phae- 

 nomena connected with them demand attention. 



I shall only here advert to one, the concentrically laminar colora- 

 tion, frequently seen on a small scale, and so splendidly developed in 

 Sahara, Jong, and Bukum. The very same zoned coloration is ob- 

 servable in some of the sections cut into decomposed plutonic rock, 

 such as that where the public road crosses Mr. Hewetson's Hill, 

 where similar zones exist on the sides of a ferruginous dyke. The 

 neighbouring solid greenstone possesses the same structure as this 

 decomposed rock, nests of hornblende, corresponding to the red 

 patches, and hornblendic lines, often very minute, to the curved 

 zones. When we examine the great developments of solid plutonic 

 rock, we find that they present similar phsenomena. The granite of 

 Pulo Ubin, for instance, when its coherence is weakened by exposure 

 to the action of the sea, exhibits connected systems of concentric and 

 variously curved laminae, in which one system is interrupted and 

 modified by another, and accommodates the shape of its curves to 

 it, precisely as we see in those of the coloured lines of the sedi- 

 mentary rocks. 



If we do not adopt the theory of the necessity of granitic fluidity, 

 may we not suppose that, as the plutonic action first invaded the 

 superincumbent strata in the ferruginous walls and veins, it operated 

 principally through them wherever they existed ; that even when 

 complete assimilation took place they still remained, however modi- 



* Before I had ascertained the connexion between the ferruginous walls in the 

 sedimentary and those in the plutonic rocks, I endeavoured to explain the phaeno- 

 mena of composition and structure presented by Pulo Ubin, in accordance with 

 the theory of congelation. See the 1 Rocks of Pulo Ubin,' loc. cit. pp. 26 to 40. 



