﻿424 ME. J. B. SCEIVENOE ON THE E0CKS OF [Aug. I9IO,. 



following Prof. Judd's description of the Krakatoa pyroxene, 1 

 therefore, I will refer to the mineral as enstatite. 



The enstatite sometimes gives fairly good prism-sections, with 

 straight extinction and low interference-colours. Basal sections 

 show the cleavages and a bisectrix (which bisectrix is doubtful). 

 The quartz is abundant, and occurs as interstitial grains between 

 the crystals of plagioclase. Magnetite is abundant, while zircon 

 and apatite are also associated with the ferromagnesian minerals. 

 The hornblende, in part at any rate, is a secondary product derived 

 from the enstatite. The specific gravity of the rock is about 2-81. 



In the centre of the section is a large and complex vein, a 

 branch of which (c) is the continuation into the face of the quarry 

 of the ' dyke-like mass ' figured on p. 20 of my previous paper, 

 in the ' Geological Magazine ' for 1909. This consists of a very 

 fine-grained, dark, reddish-brown rock, and can be followed 

 down into the mass of the vein in the base of the quarry-face. 

 A separate vein (c) of the same rock occurs to the right of this 

 big vein. The big vein itself consists of a mixture of the fine- 

 grained rock and a rock of varying coarseness, which, in the hand- 

 specimens, seems identical with that forming the veins on the right 

 of the section. Under the microscope, sections show that it is 

 of much the same composition, the points of difference being 

 the greater proportion of green monoclinic pyroxene, and the 

 predominance of hornblende and biotite over the enstatite. Quartz 

 is abundant. 



Specimens from the fine-grained rock (c) differ, in that one 

 ferromagnesian mineral may be present in one specimen but absent 

 in another. The rock, considered as a whole, may be described as 

 follows : — The base consists of plagioclase and what appear to be 

 minute prisms of orthoclase with some interstitial quartz. In this 

 are ragged greenish-brown flakes of hornblende, small plates of 

 biotite without crystal outline, and granules and ragged masses 

 (which are, however, in optical continuity) of monoclinic pyroxene 

 and rhombic pyroxene of the same nature as that in the coarser 

 rocks. The last mineral is recognizable by its low interference- 

 colours, straight extinction, and sometimes by the pleochroism. 



On the left of the section are two parallel veins of the fine-grained 

 rock (c) and several masses of the same rock, that can only be 

 interpreted as included fragments caught up by the granite. With 

 the exception of these masses, all the enstatite-bearing rocks in 

 this quarry are certainly in the form of veins. Their mode of 

 origin will be discussed later. 



A curious point about the North Quarry is that on the whole 

 of the large granite-face no ' basic patches ' were observable when I 

 visited it, unless the masses of the rock c be considered as such. 



1 'The Eruption of Krakatoa & Subsequent Phenomena ' Eeport of the 

 Krakatoa Committee of the Boyal Society, 1888, pp. 81, 32, & 34. The 

 mineral in the Pulau Ubin rocks is probably bronzite ; but, in the absence 

 of definite information as to the position of the acute bisectrix and the 

 percentage of iron, it is better to refer to it as enstatite. 



