20 Proceedings of the Royal Irish Academy. 



which left the more resistant chert to be entombed in the ash. This seems 

 a possible explanation of the fact that the larger number of pebbles found in 

 the ash are chert. 



That the igneous material, which forms the necks, actually breaks through 

 the Carboniferous limestone can be seen in several sections. In a quarry, 

 about half a mile east of the summit of Croghan Hill, dark limestone with 

 bands of shale and many crinoid stems occur. The limestone here is much 

 disturbed, and the bedding is rather obscure ; but it sometimes dips towards 

 the centre of the hill, and sometimes a little east or west of it. The limestone 

 strata are penetrated by a neck which rises a little further to the east, and 

 close to the edge of the bog, showing clearly that the igneous material has 

 been forced up through the limestone. Similarly, about three-quarters of a 

 mile south-east of this point, near Barrysbrook House, the igneous material 

 may again be seen breaking through the limestone. A like feature may be 

 observed by the roadside at Gorteen, on the south-western side of Croghan 

 Hill, where " the limestones have been thrown into a highly inclined position, 

 dipping towards the east at 60° or more, and their truncated ends abut 

 against the sides of the neck." 1 At Glenmore holy wells, one-third of a mile 

 north-north-east of the summit of the hill, black, shaly and cherty limestone 

 are seen dipping south at a low angle into the body of the hill ; a little north- 

 west from this, on the hill, grey limestone is exposed, and several other out- 

 crops of a similar limestone are seen round this point. This grey limestone 

 runs south-west in a curved band, and is well seen on the road a short distance 

 west of Gorteen. At this point a quarry is opened, and the rock is seen to 

 be a hard, grey limestone, much jointed, and has all the appearance litho- 

 logically of the Lower limestone ; but, owing to the lack of fossil evidence, 

 this cannot be definitely ascertained. 



On passing over this band of grey limestone, in a northerly direction, the 

 black, shaly limestone again appears, and is now seen to dip northwards at 

 about 20° under the ash, which covers the surface between this point and the 

 bog to the north. The dark, shaly limestone is also seen a little east of this, 

 but here it has a north-easterly dip. As mentioned above, this limestone has 

 all the appearance of the Middle limestone or " Calp," and in all probability 

 represents the basement beds of this series. This is overlain directly by the 

 ash and underlain by the compact grey or Lower limestone, the latter being 

 brought to the surface by an anticlinal fold, as the black limestone is seen 

 dipping south under the main ash on Croghan Hill and dips north under the 

 ash surrounding Boston ; the grey limestone being exposed on the surface 



1 Sir A. Geikie, " Ancient Volcanoes of Great Britain, ' vol. ii, 1897, p. 38. 



