﻿432 ME. J. B. SCRIVENOK ON THE ROCKS OF [Allg. I9IO, 



seems to me to be clearly due to difference in time of intrusion. 

 The coarse quartz-norite veins and the fine-grained enstatite- 

 spessartite are of much the same mineral composition 5 and, if we 

 assume that the veins of quartz-norite were intruded when the 

 granite was very hot, and the enstatite-spessartite later when it 

 was cooler, there is no difficulty so far as the veins are concerned, 

 since the big complex vein in the centre of the quarry lends itself 

 to the explanation that it was formed by a later eruption of 

 enstatite-spessartite along the same path as that which the quartz- 

 norite had taken. The masses of enstatite-spessartite must be left 

 for the moment. 



Passing to Quarry No. 1, where we have the same normal horn- 

 blende-granite, with basic masses (I have been careful to assure 

 myself, by visiting small Chinese quarries, that this is the pre- 

 dominant rock on Pulau Ubin, and that it may reasonably be 

 assumed to be all of the same period of irruption), these latter 

 resemble the quartz-norite veins in the North Quarry ; and, 

 although the enstatite is not abundant, one is impelled towards 

 seeking a common origin for both veins and masses. The problem 

 then is this : in one quarry we have veins of enstatite-spessartite 

 intruded when the granite was cool, and masses of the same rock 

 included in the granite ; while in the same quarry we have thin 

 veins of quartz-norite, and in another, masses of a rock resembling 

 this vein-rock. Spessartite and gabbro are both anterior to, and 

 later than, the granite. 



There is only one way of accounting for this, which is to assume 

 that prior to the irruption of the granite there were two distinct 

 magmas, one granitic, the other gabbroid, and that the granite in 

 its ascent caught up portions of previously-irrupted gabbroid 

 magma, while later irruptions of the same gabbroid magma in- 

 vaded the consolidated granite. It is possible that in Divisions 

 II & III we have a repetition of these relations ; but, in the absence 

 of clearly-defined vein-rocks, I cannot say more. If it should 

 prove later that these divisions afford a parallel case, the problem 

 will be greatly complicated. It will be remembered that some of 

 the basic masses in Quarry No. 1 contain spongy masses of horn- 

 blende like those in the dark masses of the South Quarry, and in 

 the porphyry behind the North Quarry. 



It appears certain that a gabbroid magma x had been differ- 

 entiated before the normal hornblende-granite of Pulau Ubin 

 consolidated, and there is reason for supposing this granite to be 

 pre-Eocene, if not pre-Cretaceous. It is remarkable that Dr. 

 Verbeek's description of Amboyna enables us to find a place for 



1 The amount of quartz iu some of the rocks derived from this magma is 

 i extraordinarily high for gabbro. The mean of the specific-gravity determina- 

 tions (see Geol. Mag. dec. v, vol. vi, 1909, p. 21 & this paper) is not too low, 

 however, for a gabbroid magma, and the felspar is mostly andesine-labradorite. 

 In view of the variable amount of free silica in the rocks, silica-percentages 

 would be sure to vary as much as the specific gravity. 



