267 



individuals are frequently twinned. They are partly idiomorphic and 

 partly ophitk-. The biotite like the hornblende varies in colour from 

 brown to green. Sections at right angles to the principal cleavage often 

 show alternating lamellae of the brown and green varieties. The felspar is 

 idiomorphic with respect to the quartz. Lath-shaped plagioclase is often 

 seen lying in a matrix of quartz. Apatite is present as usual A similar 

 rock but with less quartz occurs at Kilmacreagh in the same county. 



A rock which must be referred to the quartz-diorites is extensively 

 developed in the ridge, coloured syenite and greenstone, which runs east 

 and west about three miles S. of Haverfordwest in Pembrokeshire. It is well 

 exposed in a quarry near Targate. It is distinctly crystalline and dark 

 grey in colour. A similar rock occurs at Talbenny. An interesting feature 

 connected with this rock Is that it is intimately associated with a light 

 coloured granite which is frequently seen cutting it in the form of veins. 

 Both jrranite and quartz-diorite are cut by highly decomposed dykes of 

 basic trap 'diabase . The principal constituents of the diorite are horn- 

 blende, felspar, quartz, iron-ores and epidote. The hornblende is green in 

 colour and occurs in the form of irregular grains. The felspar is generally 

 much altered. Twin-striation may be frequently seen but it is not common 

 even in the least altered rocks. Like the hornblende it occurs for the 

 most part in irregular grains. In the most altered specimens grains oi 

 epidote are common in the felspars. Quartz is tolerably abundant, also in 

 the form of irregular grains. Iron ores occur sparingly. The texture of 

 the rock is granitic and no mineral appears to have obtained any very 

 decided advantage over any other so far as the development of crystalline 

 form is concerned. Here and there the felspar may be seen to be idio- 

 morphic with respect to the hornblende. In some places the quartz-diorite 

 may be seen to assume a gneissic structure in the neighbourhood of 

 dislocation planes. 



Yen- typical diorites occur in the CTiannel Islands, Guernsey and 

 Jersey. Those from the former island have been described by Messrs. 

 HILL and BONNET <*>; those from the latter by M. NouBY.< 2 > A specimen 

 from St. Clements, Jersey, is a medium-grained greyish rock of granitic 

 texture, very similar in character to the more felspathic varieties of the 

 Glen Tilt diorite. It is composed of felspar, mostly showing twin-striation, 

 hornblende, biotite, quartz, iron-ores and apatite. The hornblende often 

 shows traces of crystalline outline but it is frequently penetrated by lath- 

 shaped felspars. The felspar is idiomorphic with respect to the quartz. 

 The broad lath-shaped felspar sections are sometimes seen to be sharply 

 terminated by crystalline faces where they penetrate into a large irregular 

 individual of quartz. The hornblende varies in colour from brown to 



ii. Biotite occurs sparingly. It is often changed to chlorite, in which 

 epidote grains are scattered. Iron-ores and apatite occur sparingly. 



Great difficulty is experienced in deciding on the boundary line between 

 the quartz-mica-diorites and the hornblende-biotite-granites or hornblende- 



(1) Q.J.G.S., Vol. XL. 1884), p. 404. 



(2) Geologic de Jersey. Paris, 1886. 



