﻿600 ME. J. V. ELSDEN ON THE IGNEOUS ROCKS [Aug. I905, 



Apatite. — This mineral is a very prominent constituent of 

 certain of the rock-types, but in others it is rare or absent. It is, 

 for example, extremely rare in the Fishguard and Prescelly diabases, 

 and in the more basic variety of the enstatite-diabases of the Llan- 

 rian and Trevine areas ; but in the quartz-enstatite-rocks it is often 

 abundant, and always present to some extent. This distribution of 

 apatite is very noticeable at Cam Llidi. The basic norite of the 

 western extremity of this mass apparently contains no apatite, but 

 towards the east, where the rock passes into quartz-norite, apatite 

 becomes conspicuous. In the lime-bostonite series it seems to be 

 wanting. 



Iron -ores. — Magnetite seems to be confined almost entirely to 

 some of the fine-grained rocks of Strumble Head, the dolerite of 

 Cam Gilfach, and the fine-grained intrusions of Y Garn. In all 

 the other types the only recognized species of ironrore is ilmenite, 

 often in good rhombohedral forms. The usual decomposition- 

 products, leucoxene and sphene, are present. Pyrites is often 

 present, but is apparently not confined to any particular groups of 

 rock. 



Rhombic pyroxen e. — The rhombic pyroxene occurs in vari ous 

 forms, and exhibits very different degrees of alteration. The 

 least- altered forms occur in some of the diabases, and show no fibrous 

 structure. Only the pinacoidal cleavages are recognizable. These 

 possess marked pleochroism : rays vibrating parallel to the y axis 

 are green, those vibrating parallel to the (3 axis being yellow. 

 In no case is the brownish-red colour, characteristic of the brachy- 

 diagonal vibrations (a) in typical hypers thene, visible. These 

 characters would seem to point to a bronzite-variety of enstatite. 

 Various degrees of hydration lead to the development of the fibrous 

 forms, diaclasite and bastite, with a corresponding loss of pleo- 

 chroism ; while, in other cases, the mineral passes into a kind of 

 chloritic pseudomorph. In the diorite-series the prevailing form of 

 the enstatite is in prisms terminated at one extremity by rather 

 rounded domes. The crystals frequently are elongated in the 

 direction of the c axis. They are always fibrous and generally 

 faintly pleochroic, the latter quality being sometimes confined to 

 patches only. 



Lastly, in the norite-series we find the rhombic pyroxene in broad 

 plates, often changed to a colourless fibrous substance with high 

 double refraction and nearly-straight extinction. This may be an 

 amphibole. 1 At other times the ordinary bastite-alteration is 

 present. All the varieties include sphene-granules. 



Interesting variations occur in the order of separation of the 

 rhombic pyroxene. In the diabases it seems always to have preceded 



1 This change has been noted in other cases; see O. H. Erdmannsdorffer, 

 'Die devomschen Eruptivgesteine & Tuffe bei Harzburg, &o.' Jahrb. der 

 k. Preuss. Geol. Landesanst. & Bergakad. toI. xxv (1904) pt. i, p. 47. 



