Haigh — Carboniferous Volcanoes of Philipstotvn, King's Co. 31 



as reddish brown pleochroic flakes, which, though it occurs in isolated patches, 

 is usually seen aa a mantle round the iron ore grains which are scattered 

 abundantly in the rock. This intrusion appears to belong to the lamprophyre 

 group, and might be described as a mica-augite-lamprophyre. It has a 

 specific gravity of 2-88. 



The ash over the whole area is of a very uniform character, having a 

 greenish colour, and enclosing fragments of chert, limestone, and basalt. The 

 fragmentary material for the most part is a highly vesicular pumice, the 

 vesicles being generally filled with calcite. The pumiceous fragments are 

 angular, sub -angular, and rounded, and are set in a calcareous cement, which 

 is now generally represented by white calcite, but which undoubtedly consisted 

 in large part originally of limestone. On weathering, the rock often gives 

 rise to a curious mosaic structure due to projecting fragments of pumice. In 

 some instances the calcareous cement is stained red with iron oxide. In the 

 calcite material between the fragments, spherulitic areas occur, in which lie 

 a number of microlites of felspar in an almost opaque white ground. In 

 some of these areas the microlites lie haphazard, but in others crystallization 

 seems to have taken place from the outer rim, 

 the microlites radiating towards the centre 

 (fig. 8). The spherules are probably concretions, 

 similar to those described from a tuff from 

 Torres Strait. 1 



In the Irish examples the spherules are 

 probably composed of complex lime silicates, 

 from which the felspar has crystallized as a 

 product of secondary change. On the south- 

 west side of the hill, near Gorteen, a small 

 outcrop of a tuff appears, which differs in 

 character from the main mass. It is a brittle 



greyish rock, containing patches of a slightly altered brown glass. In 

 section these brown patches have a flecked appearance, which is probably 

 due to minute enclosures of felspathic material. The interstices between the 

 glass is filled in with felspar in which secondary crystallization has taken 

 place. The rock is a palagonite tuff. Just north of the summit of Croghan 

 Hill, the ash is of a very compact nature, with a bluish-green colour. It 

 contains many joint planes along which it splits very readily. These planes 



1 Haddon, Sollas, and Cole, " Geology of Torres Strait." Trans. Royal Irish Academy, 

 30, p. 419 : cf. I. Friedlaender, " Uber die Kleinformen der vulkanischen Produkte. " 

 Zeitschrift fur Vulkanologie, Band I, Heft I, Jan. 1914, p. 37, fig. 13. 



