Yol. 56.] IGNEOUS ROCKS OF COUNTY WATERFOKD. 685 



group of the smaller intrusive veins might lead one to attribute 

 them at first to the dolerites or andesitic dolerites; and some 

 are in so decomposed a condition that even sections under the 

 microscope do not prove of much assistance in determining their true 

 character. 



c. The Rocks of the Smaller Intrusive Veins. 



Several more or less distinct types are recognizable under the 

 microscope in this somewhat mixed assemblage. Most of the rocks 

 are in a highly decomposed condition, and their original characters 

 are hard to distinguish. Many may be ascribed to the andesitic 

 trachytes, but possibly some of these should rather be classed 

 with the keratophyres, as suggested to me by Mr. Henry 

 Seymour, of the Geological Survey of Ireland, who kindly examined 

 a few slides. In macroscopic character these rocks vary from light- 

 grey to dark-green, are opaque in appearance, and have a rough 

 fracture. 



A large number of microscope-sections have been examined 

 by me. The predominant constituent is felspar, which occurs 

 in lath-shaped crystals of generally perfect crystalline form, and 

 when sufficiently fresh they indicate by their optical characters 

 that they belong to the albite or oligoclase-andesine group. 

 The ferromagnesian elements are not abundant, and have usually 

 been replaced by chlorite and other secondary products, such as 

 calcite, epidote, etc. Frequently augite seems to have been present, 

 and to have been idiomorphic. Magnetite occurs in grains or rods, 

 but ilmenite or leucoxene is often observable as the predominant 

 iron-ore. Quartz-grains are not uncommon. The groundmass 

 is sometimes abundant, and contains the generation of smaller lath- 

 shaped crystals or microlites of felspar. 



In many the trachytic character is more marked (Doneraile Cove, 

 etc.), but in others near Sheep Island the larger felspars are less 

 numerous, while the groundmass is microcrystalline and more abun- 

 dant, so that these rocks might be classed with the more acid felsites. 



In others (Newtown Head, Passage) the groundmass consists of 

 numerous small short felspar-crystals, with comparatively few 

 larger felspars, and occasionally the latter are almost completely 

 absent (Knockmahon). In these last-mentioned examples the 

 ferromagnesian constituents are very rare. There are many examples 

 of this type along the Waterford coast, and some of them show signs 

 of flow by the parallelism of the felspar-microlites (Newtown Head, 

 Passage, and Bunmahon). Some have skeleton felspar-crystals 

 (eastern side of Garrarus Strand). 



There are a few of these smaller intrusive veins in which the 

 groundmass is micro- or cryptocrystalline, and no ferromagnesian 

 minerals are apparent. These are rather rhyolitic than trachytic 

 felsites (Sheep Island). A fine microlithic felt occasionally forms 

 the groundmass (Foilnaneena, Garrarus, Kilfarrasy, fig. 6, p. 664), 

 and in some cases these rocks contain small phenocrysts of pyroxenes : 

 they should perhaps be assigned to the andesites. In a curious 

 banded example with flow-structure from Garrarus the lighter 



