J. B. Scrirenor — Igneous Rocks of Singapore. 21 



a way that it was impossible, viewing this mass alone, to tell which 

 rock was the later. 



This rock proved very interesting in section. There are three 

 ferromagnesian minerals in a clear base consisting of small grains of 

 orthoclase, qnartz, and a little plagioclase. They are biotite (the 

 least abundant), brownish-green liornblende in ragged flakes (abundant), 

 and numerous granules and ragged prisms of rhombic pyroxene, 

 showing the same pale-green and pale-carmine pleochroisra as the 

 mineral in the rocks described above. 



Traversing this dark I'eddish brown rock the writer found a small 

 white and pale-brown vein, which on examination proved to consist of 

 grains of pink garnet, pale-green monoclinic pyroxene, wollastonite 

 (largely altered to calcite), and a little quartz. 



At Changi large masses of a tine -grained rock {d) were found in the 

 granite resembling the reddish-brown rock fi'om Pulau TIbin, but 

 under the microscope the resemblance proves to be deceptive. It 

 consists of orthoclase in short, well-formed prisms, a little plagioclase, 

 abundant interstitial quartz, which is in optical continuity over large 

 areas of the field, a little biotite in small ragged flakes, green 

 hornblende, and abundant apatite. 



"Basic patches" are rare in the Pulau TJbin granite. Those that 

 do occur are large and angular, and slides were cut fi'om a good 

 example {e) (in quarry No. 1) with a view to comparing it with the 

 rocks just described. It proved to consist of plagioclase with some 

 orthoclase, abundant apatite, and equal quantities of biotite, pale- 

 green monoclinic pyroxene, and green hornblende, part of which at 

 any rate is derived from the pyroxene. 



The following determinations of specific gravity have been made : — 

 Hornblende ,ffranite troui Ihe Municipality Quarry . . . 2-68 

 Dark hornblende granite from Tanjong Jeluton^ . . .2-71 

 The rock a from the Municipality Quarry .... 2'89 



The rock b from the Municipality Quarry .... 2-83 



The rock c from the Municipality Quarry .... 3-01 



The rock <? from Changi . . . . . . .2-76 



The basic patch e from quarry I.'o. 1 .... . 2"88 



It was thought at the time of collecting the specimens that a and b 

 were but variations of the granite due to local differentiation, and 

 there was no field evidence to show that they were of a different date 

 of intrusion. But the fine-grained rock c is, in part at any rate, of 

 later date than the granite, and the occurrence of the rhombic pyroxene 

 in ff, b, and c, and its (apparent) absence ' from the granite and the 

 basic patches, make it probable that this view was incorrect. 

 Supposing, however, that it be granted that the rocks a, b, c all 

 . come from a separate magma, a further complication is introduced by 

 the absence of rhombic pyroxene in the fine-grained rock d at Changi. 



Granite containing rhombic pyroxene is not unknown in the Malay 

 Archipelago. Mr. AVing Easton, describing the granite of West 



' A quantity of granite, from the part of tlie quarry where the rock o is descril)ed 

 as being mingled with it, was crushed, and the heavy minerals concentrated. Among 

 them a few grains of the rhombic pyroxene were found, but this cannot be accepted 

 as conclusive proof of the rhombic pyroxene being a constituent of the granite, as it 

 may possibly have come from the other rock. 



