﻿Yol. 64.] ST. DAVId's-HEAD ' KOCK-SERIES.' 283 



{d) The Aplite- Veins. 



There are also some fine-grained veins which penetrate the acid 

 type of the main rock. These are very thin, and by no means 

 common. The specimen selected for analysis occurs near the 

 northern margin of the Carn-Llidi mass, not far from Pen Lledwen. 

 Under the microscope it is seen to consist mainly of felspar and 

 quartz, with some disseminated iron -oxide, but without any ferro- 

 magnesian minerals. The felspars have square-shaped idiomorphic 

 outlines and are usually rather turbid, but occasionally they show 

 plagioclase-striation. The quartz is interstitial, and no micro- 

 pegmatite is visible. The junction with the coarser gabbro seems 

 to be well-defined in hand-specimens, but under the microscope 

 it is less definite, the aplite passing gradually into the intersertal 

 material of the coarser rock. It is evidently, therefore, of contem- 

 poraneous origin. These aplite-veins have not been noticed in the 

 more basic portions of the mass, but may nevertheless occur. 



A chemical analysis of this rock gave the following result : — 



SiO. 



Fer cent. 

 71-18 



Molecular 

 proportions. 



1186 



TiO, 



ALO, 



0-48 



1489 



144 

 6 



I:??!.::;:::;;:: 



CaO 



MffO 



211 



1-21 



0-82 



014 



13 



16 



14 



3 



Na^O 



6-85 



112 



K,0 



H2O- 



1-70 



0-24 



18 



H,0+ 



064 









Specific gravity = 



Total 



100-26 



The above composition admits of the following mineral propor- 

 tions : — 



Quartz 22-68 



Orthoclase 10*01 



Albite 58-69 



Anorthite 3-90 



Magnetite and ilmenite 3-22 



Total 98-50 



In apportioning the molecules there is a small excess of alumina, 

 ferric oxide, and magnesia, which is probably accounted for by the 

 presence of hydrated decomposition-products of the felspar, by 

 which the free silica would of course be proportionately reduced. 

 The rock is evidently a soda-aplite (see PL XXIX, fig. 6). Since 

 no free orthoclase can be detected microscopically, this mineral 

 is possibly included in the albite -crystals, either in solid solution 



