﻿426 



ME. J. B. SCEIVENOR ON THE ROCKS OF [Aug. I9IO, 



^?m^§ 



enclosed in the granite and none of them intrusive. Specimens 

 from several masses were collected, and the slides prepared from 

 them reveal an unexpected resemblance to the veins ab and 

 the coarse portion of the big vein in the North Quarry. 



The following general description will illustrate this : — The rock 

 composing the basic masses is crystalline and of medium grain, and 



the proportion of the 



;. 3. — Angular masses of quartz-biotite-gabbro in dark minerals varies 



the hornblende-granite of M. 1 Quarry, Pulau slightly. These dark 



JJbin. minerals are biotite, 



green hornblende, pale- 

 green monoclinic py- 

 roxene, and sometimes 

 rhombic pyroxene also 

 (like the enstatite de- 

 scribed above, but 

 largely altered to green 

 hornblende), accom - 

 panied by apatite, zir- 

 con, and magnetite. 

 In some of the speci- 

 mens it was noted that 

 the hornblende occurs 

 as spongy masses re- 

 sembling those in the 

 porphyry exposed be- 

 hind the North Quarry. 

 The apatite, in long 

 prisms, is sometimes 

 very abundant, and the 

 crushed rock gives a 

 heavy precipitate when 

 a nitric-acid solution 

 is treated with ammo- 

 nium molybdate. 



The clear minerals 



in these basic .masses 



are plagioclase (like 



that in the quartz- 



norite of the North 



Quarry), perhaps a 



little orthoclase, and 



interstitial quartz, the amount of which varies considerably in 



different specimens. These masses are, then, of very similar 



composition to the coarse vein-stones in the North Quarry, and may 



be described as quartz-biotite-gabbro with subordinate 



enstatite. 



