﻿294 THE ST. DAVID 's-HEAD ' EOCK-SERIES/ [MajTlQoS, 



Fig. 4. Eock from Trwyn Llwyd, showing fluxion-structure. Crystals of 

 rhombic and monoclinic pyroxene, i^lagioclase, and opaque iron-ore 

 are elongated in the direction of flow. (X 20 diameters.) See 

 p. 277. 



5. Rock from Porth Melgan, showing idiomorphic pyroxenes. (X 20 



diameters.) See p. 276. 



6. Specimen from the central ridge of the St. David's-Head mass, showing 



felspar-alteration. Near the right-hand margin a plagioclase-crystal is 

 seen, partly replaced by quartz and a mineral presumed to be prehnite. 

 (X 20 diameters.) See p. 285. 



Plate XXXI. 



Pig. 1. Large rhombic pyroxene, typically developed in the basic varieties. 

 The specimen is from Carn Llidi. (X 15 diameters.) See p. 286. 



2. A similar crystal, enclosing felspars. (X 15 diameters.) See p. 286. 



3. Intermediate type, showing rhombic and monoclinic pyroxene, and 



plagioclase. The order of crystallization is here plainly revealed. 

 The large augite-crystal on the right exhibits basal striation. (X 15 

 diameters.) See p. 286. 



4. A large crystal of rhombic pyroxene occupies the bottom of the field, 



plagioclase the centre, and monoclinic pyroxenes, with basal striation, 

 the top. ( X 15 diameters.) See p. 287. 



5. On the left a large crystal of augite, with basal striation, shows partial 



schillerization. The remainder of the field is occupied by plagioclase, 

 and chloritic alteration-products. (X lo diameters.) See p. 287. 



6. The greater part of the field is occupied by a large crystal of augite 



with basal striation, optically intergrown at the bottom with an 

 unstriated augite-crystal. (X 15 diameters.) See p. 287. 



Plate XXXII. 



Fig. 1. The upper margin of the slice shows an interlocked twin-crystal of 

 rhombic and monoclinic pyroxene, the composition-plane being the 

 orthopinacoid of the latter. The section is cut approximately 

 parallel to the clinopinacoid, the augite extinguishing at about 35^ 

 to the twinning-plane, ( X 10 diameters.) See p. 286. 



2. At the bottom of the field is a twin-crystal, consisting of rhombic and 



monoclinic pyroxene. The remainder of the field is made up of 

 rhombic and monoclinic pyroxene, plagioclase, and quartz. (X 15 

 diameters.) See p. 286. 



3. In the top left-hand corner is a twin of rhombic and monoclinic 



pyroxene cut transversely. The rest of the field contains plagioclase, 

 augite developed into enstatite, and opaque iron-ore. (X 20 

 diameters.) See p. 286. 



4. This shows parallel development of rhombic and monoclinic pyroxene, 



in the upper portion of the field. (X 15 diameters.) See p. 286. 



5. This shows three twins of rhombic and monoclinic pyroxene, all cut 



transversely to the c axis. ( X 15 diameters.) See p. 286. 



6. Section illustrating reversed order of crystallization, pyroxene preceding 



felspar. (X 20 diameters.) See p. 286. 



Discussion. 



Dr. J. S. Plett said that this paper was of the greatest interest 

 to him, as he had had occasion to investigate two sets of rocks 

 which had many points in common with those described. By 

 whatever names they might be called — quartz-norites, quartz- 

 gabbros, hypersthene-diabases, quartz-diabases, sahlite-diabases, 

 quartz-dolerites, etc. — they all consisted of rhombic pyroxene, 



