Hyland — On some Specimens from Wad y- Hal/a. 443 



whether we are dealing with a felspar of the anorthoclase series it 

 is impossible to say. 



Both mica and hornblende are present. The inclination is to 

 develope a rod-shaped form. The colour of the two minerals is 

 practically the same, being green to greenish yellow. The direc- 

 tions of extinction and the degrees of pleochroism serve to distin- 

 guish the one from the other. The maximum extinction angle of 

 the hornblende fC : c) was found to be 14°, and the pleochroism 

 deep green to rich yellow. There is twinning parallel to 100. 

 Chlorite is present, and appears to be due to the alteration of the 

 mica or hornblende. A vein of epidote runs through the rock, 

 and secondary quartz can also be observed. Primary quartz is 

 also to be noticed, and contains inclusions of a colourless, needle-like 

 mineral, probably apatite. The amount of quartz, both primary 

 and secondary, is not inconsiderable. Carbonates, sphene, and 

 iron ores are not absent. 



The microscopic structure is that of a holo-crystalline rock. 

 There is no trace of foliation ; and the dykes of this series are 

 evidently posterior to the earth-movements which have affected 

 the crystalline schist area. We have applied the term Lam- 

 prophyre to denote the rock. This name was first introduced 

 by Giimbel, 1 to define rocks which occur as a series of dykes 

 (usually running N. & S.), in the Palaeozoic strata of the Fichtel- 

 gebirge, Thiiringerwald, and Voigtland in Germany. They occur 

 as high as the Culm Measures, are mostly dark-coloured, and bear 

 a strong resemblance to " diabase," though they differ from that 

 rock in containing much dark mica. Manifestly, Giimbel's Lam- 

 prophyres are similar in composition, geological age, and mode of 

 occurrence, to the mica- traps of Cornwall and Devon. 2 Rosen - 

 busch 3 extends, however, the signification of Giimbel's term, and 

 distinguishes between the syenitie and dioritic Lamprophyres ac- 

 cording as orthoclase or plagioclase is the dominaut felspar. The 

 general name, " Lamprophyre," he applies to rocks which occur 

 as dykes in disturbed regions, and which so far as the composi- 

 tion is concerned, have affinities on the one hand with syenites, 



Guognostische Besehreibung des Fichtelgebirges, p. 189. 



Teall, British Petrography, p. 351. 



Mikroskopisohe Physiographie der niassigen Gesteine, 1S87, p. 308. 



