684 ME. E. K. COWPER REED ON THE [Nov. I9OO, 



deserve comparison, and Mr. Harker 1 has remarked that some of 

 these closely resemble some of the rocks called bostonite. It is 

 probable that many of the Water ford felsites with the abundant 

 phenocrysts of plagioclase (albite) are soda-felsites. 



b. The Diabases and Dolerites. 



All these rocks were classed as ' greenstones ' in the Geological- 

 Survey maps and memoirs, but they show considerable diversity in 

 structure and composition. In County Wicklow greenstones and 

 basic sills of probably the same age have been described by Mr. Teall, 2 

 Dr. Hatch, 3 and A. von Lasaulx. 4, While s o m e of them are diabases 

 others are varieties of diorite, and some of the Waterford 

 rocks may belong to the latter category. Most of the 

 rocks are considerably decomposed. 



Some of the Waterford basic sills show true ophitic structure 

 (Tramore Bay, etc.) ; but in others, though the felspars have 

 crystallized out earlier than the ferromagnesian minerals which 

 mould them, they cannot be described as truly ophitic (Little 

 Island, Great Newtown Head). The augite is usually replaced 

 by some chloritic mineral, frequently of a bright green colour, but in 

 other cases it is nearly or quite fresh. Some of them have portions 

 of the unindividualized groundmass containing small felspar-laths 

 between the felspar-crystals (Newtown Head, Passage) ; but in 

 other parts of the same rock-mass this may be entirely absent and 

 the structure almost holocrystalline, all the constituents being allo- 

 triomorphic, and apparently having crystallized out simultaneously. 



Many of these diabases contain quartz (Tramore Bay), and in 

 some cases it appears to be original. Apatite is a common con- 

 stituent, and ilmenite or leucoxene is always present in skeleton- 

 crystals, large plates, or small grains. The felspars are mostly 

 varieties of labradorite, but are as a rule much decomposed. Some 

 of the so-called diabases might more correctly be ascribed to the 

 diorite family (Garrarus, Tramore), so far as their structure goes, 

 but in no case has hornblende been detected, its place being taken 

 by augite or its decomposition-products. The holocrystalline 

 appearance of these rocks under the microscope, and the absence of 

 ophitic structure and of all interstitial material, suggest that they 

 might be put among the augite-diorites which are so well 

 developed in Wicklow. Some contain quartz. 



The dolerites are very few, and call for no special mention. Two 

 from the neighbourhood of Great Newtown Head are of the nature 

 rather of andesitic dolerites. The augite is occasionally found 

 in fairly fresh porphyritic crystals, but is generally intersertal and 

 granular. The felspars are generally much decomposed, and iron- 

 ores are usually abundant. 



The appearance in the field of many of the rocks belonging to the 



1 ' Petrologv for Students ' 1895, p. 150. 



2 « British Petrography' 1888, pp. 249 & 266. 



3 Mem. Geol. Surv. Irel. 1888, Expl. Sheets 138 & 139, p. 43; Geol. Mag. 

 1889, p. 261. 



4 Tscherm. Min. u. Petr. Mitth. vol. i (1878) p. 441. 



