Haigh — Carboniferous Volcanoes of Philipstoivn, King's Co. 25 



sillimanite. This appears to be a metamorphic rock, and may be a fragment 

 of some sediment enclosed in the igneous rocks." From an examination of 

 one of these rock-sections it appears to be a fragment of a sedimentary rock 

 which has been broken off and enclosed by the lava in its ascent. The 

 quartz crystals contain numerous cracks which are filled with a brown glass. 

 The liquid mass was evidently forced into the interstices of the rock and into 

 the cracks in the crystals. In this feature the rock nraeh resembles that in a 

 dolerite dyke from Ross Harbour Point on the shore of Lower Lough Erne. 1 

 The interest of these rocks is obvious, as giving some indication of the character 

 of the floor of the country below the Carboniferous and, perhaps, below the 

 Silurian strata. 



The various outcrops of the intrusive rocks will now be described in 

 detail, beginning with the more northern outcrops. About three quarters of 



a mile north-north-east of the cairn on the 

 summit of Croghan Hill, a circular-shaped knoll 

 rises about two hundred feet above the level 

 of the plain. It is surrounded on the north, 

 west, and south sides by ash, which in turn is 

 surrounded by limestone, the limestone abutting 

 directly against the intrusive rock on the 

 western side. This is a dark-blue compact, 

 slightly amygdaloidal rock, the amygdales being 

 composed of calcite (fig. 3). It is fine-grained 

 and crystalline, and it shows good idiomorphic 

 crystals of augite set in a matrix of smaller 

 crystals. The pyroxene, which occurs in two generations, has generally the 

 purplish tinge which is taken as an indication of the presence of titanium. 

 The larger crystals sometimes show zonal structure, and a gradation in colour, 

 being brown or purple on the outside, and fading away to a perfectly clear 

 pyroxene at the centre. Much olivine was originally present in the rock, both 

 as crystals and irregular blebs, but it now only exists as pseudomorphs in 

 calcite and chlorite, and often presents a mosaic structure, the cracks of which 

 in some cases still show traces of iron oxide. Some of these pseudomorphs 

 have a perfect olivine outline, while others occur as irregular grains and 

 patches scattered through the ground. ISTo fresh olivine was seen in the rock. 

 Much titaniferous magnetite was originally present, but it is now mostly 

 altered into leucoxene. A few crystals of apatite were also observed. Amyg- 

 dales occur filled with calcite, which show a radial structure round the edges, 



Fig. 3.— x 



1 Geol. Surv. Ireland, Explan. Mem., Sheet 32, pp. 43 and 21. 



E.I. A. PROO., VOL. XXXII., SECT. B. 



IE] 



