﻿1851.] 



LOGAN — GEOLOGY OF SINGAPORE. 



339 



should see in the walls of the sedimentary rocks the ordinary matter 

 intermixed with pyrites and other unperoxidized forms of iron-ore, 

 and, in those of the plutonic rocks, similar forms of iron, accompanied 

 probably by a predominance of those common plutonic minerals in 

 which iron is a large ingredient, owing to its accumulation there at 

 the time of the crystallization. The ready decomposition of iron- 

 pyrites would account for the great depth to which the walls and 

 veins have been peroxidized. 



17. It is of great importance to determine, if possible, the state 

 in which the iron was originally deposited, and why it was given off 

 so abundantly by the plutonic intumescence. It appears probable 

 that it was volatilized in combination with sulphur. 



Is its abundance owing to the ferruginous character of the old land 

 out of which the plutonic rock was formed, and do the highly horn- 

 blendic portions of this rock indicate a more than ordinary prevalence 

 of iron in their parent sedimentary rock ? The chocolate-coloured 

 shale-band would probably produce such a granite. Sandstone ap- 

 pears along the western boundary between the granite and this shale ; 

 but near the present Arrack Distillery (33), where they are in contact, 

 the granite is very ferruginous, decomposing into a dark-red soil, and 

 containing half-decomposed masses approaching to iron-ore. 



18. I have searched for the continuation of the ferruginous walls 

 of the sedimentary rocks into the plutonic rocks. Unfortunately the 

 line of junction of the two formations has not anywhere been laid 

 bare, all the points exposed to the action of the waves being exclu- 

 sively of the one or the other, and the deep superficial bed of earth 

 conceals the undecomposed rocks in the interior. Last year a cutting 

 of a few feet at a point (the south angle of Sri Menanti nutmeg-plan- 

 tation at the junction of the River Valley and Tanglin roads (39)) 

 where the formations meet, exposed a portion of a stratum of coarse- 

 grained sandstone lying in an irregularly convex form on the decom- 

 posed plutonic rock, which appears to have been syenite. This has 

 covered it in some places, and probably the whole or the greater part 

 of the mass was originally imbedded in it. The surfaces are irregular 

 and uneven, but have a general correspondence with the plane of the 

 bed. It varies in thickness, having in some places 4 or 5 feet of 

 well-preserved sandstone, while in others it shows only patches and 

 ferruginous plates in the syenitic clay. It varies from a fine sand- 

 stone to a coarse grit, and some parts are conglomeratic. In some 

 places it has been converted into a mass of hard and difficultly fran- 

 gible iron-ore. Where least altered the rock is hard but brittle ; the 

 granules and pebbles of quartz are all preserved, but they have a dull 

 whitish and yellowish colour, and rest in a basis of hydrous peroxide 

 of iron, varying in colour from dark-brown to black. The microscope 

 shows this basis to be not compact but vesicular, fine scales or plates 

 surrounding the quartz-granules and partly filling the interstices be- 

 tween them. Where the clay meets the sandstone, the latter has 

 generally a continuous crust or plate of shining, black, compact ore. 

 Irregular systems of a similar crust are sometimes seen with the clay 

 between them. At other places the whole is cellular, and a perfect 



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