244 MESSES. T. C. CANTKILL AND H. H. THOMAS ON THE [May 1906* 



an indistinct flow-structure may be traced in the groundmass, 

 especially around some of the larger phenocrysts. 



(/) The Diabase of Tre-hyrn. 



Specimens [E 4131] of this rock collected from well inside the 

 mass, show a moderately-fresh diabase, of a dark-green colour, in 

 which the constituent minerals are visible to the unaided eye. 

 Under the microscope, it is seen to be coarsely ophitic, and shows 

 large crystals of an almost colourless augite enclosing, and penetrated 

 by, felspar-laths with low symmetrical extinction-angles. The 

 felspars are seemingly of a composition intermediate between 

 oligoelase and andesine, and similar to those of the first generation 

 in the augite-andesites. 



The augite is in general partly decomposed, especially along 

 the cleavages, giving rise to chlorite, serpentinous material, and 

 calcite, which minerals also form irregular patches in other parts of 

 the rock. The felspars are clouded, and partly decomposed with 

 the formation of epidote, or calcite and secondary quartz. (See 

 PI. XXVI, fig. 2.) Beautiful skeleton-crystals of ilmenite occur 

 irregularly distributed through the rock, and, owing to their partial 

 decomposition into leucoxene, they appear of a dirty-white colour 

 when viewed by reflected light. 



Other specimens [E 4129] nearer the margin of the intrusion 

 show an almost complete absence of ophitic structure. In these the 

 augite seems to be of two generations: the first building fairly -large 

 idiomorphic crystals, and the second occurring as granules in the 

 groundmass. The felspars are the same as in the other part of the 

 rock, and are clouded by decomposition-products, while the larger 

 crystals are considerably epidotized. 



The diabase of Tre-hyrn has a distinct tendency, when traced 

 towards its margin, to pass from the ophitic to the granular type, as 

 is so often the case with intrusions of this character; while, in 

 hand-specimens, the rock becomes quite fine-grained, and is then hard 

 to distinguish from the fine-grained pyroclastic rocks belonging to 

 the andesitic series into which it is intruded. 



(g) The Diabase of Pentre-newydd. 



This rock undoubtedly consolidated as an ophitic diabase, but 

 now the original ferromagnesian mineral has disappeared, and its 

 place has been taken by much calcite, dusty iron-ores, serpentine, 

 and chlorite. The felspars appear to be oligoclase-andesine, and, so 

 far as their decomposed state will allow us to judge, compare in 

 composition with those in the Tre-hyrn rock. 



The pseudomorphs after the ferromagnesian mineral, which 

 consist largely of calcite, with a development of leucoxene along the 

 traces of the cleavages, are clearly ophitic in character; but the 

 cleavages are those of an amphibole, and not of a monoclinic 



