Haigh — Carboniferous Volcanoes of Philipstown, King's Co. 27 



conditions, and that the external zone, which contains the crystal faces in 

 contact with the matrix, was added during the general crystallization of 

 the magma. These aggregates belong to the category of " enclaves 

 homoeogenes," described by Lacroix, or, as Harker 1 prefers to call them, 

 " cognate xenoliths." They represent an intratelluric crystallization, which 

 would have produced a more or less coarse-grained rock, had it been 

 prolonged. Although these patches and the matrix have crystallized from 

 the same magma, they differ slightly in mineralogical composition, 

 "progressive crystallization being itself a process of differentiation." 2 The 

 rock contains very little felspar, and this only in microlites, which are too 

 small for determination of species. It has a specific gravity of 3 - 02, and is 

 a basic type of altered olivine dolerite. At the northern extremity of this 

 neck the rock is somewhat similar. It is more altered, less basic, and the 

 ground consists of a brown, altered glass, in which lie porphyritic crystals 

 of purple augite, with many smaller crystals. There is less olivine, which 

 is recognized as pseudomorphs iu calcite and chlorite. Dark brown biotite, 

 which changes to a pale brown when the polarizer is rotated, occurs as 

 scales generally adherent to the irregular crystals of titaniferous oxide, the 

 latter being nearly all altered into leucoxene. The rock differs from the 

 preceding chiefly in the quantity of felspar, which is fairly abundant. In 

 one or two instances it occurs in lath-shaped crystals, exhibiting typical 

 lamellar twinning, but extinguishes too indefinitely to permit of identi- 

 fication. It is generally present as irregular areas with no definite 

 boundaries, enclosing idiomorphic crystals of augite and magnetite. The 

 indefinite boundaries of these areas have a notched appearance due to the 

 crystals of augite and magnetite protruding into the felspar. Apatite is 

 common in long, hexagonal needles, which pierce the other constituents. 

 Calcite, chlorite, leucoxene, and limonite are the principal secondary 

 products. 



About two hundred yards north of the cairn on Croghan Hill, a rudely 

 oval-shaped area of intrusive rock occurs. One specimen 3 is a highly vesi- 

 cular rock. The matrix, now much altered, was probably originally glass, 

 with grains and skeletons of magnetite in an isotropic base. Olivine occurs 

 as pseudomorphs, these presenting a mosaic structure, and sometimes includ- 

 ing glass. The most striking feature of the rock is the numerous oval- 



1 A. Harker, " Natural History of Igneous Rocks," 1909, p. 348. 



2 Op. cit, p. 348. 



3 I am indebted to the Director of the Geological Survey of Ireland for giving me an 

 opportunity of examining this rock-section, and also the volcanic rocks collected in the 

 Limerick district. 



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