Haigh — Carboniferous Volcanoes of Philip stotvn, King's Co. 21 



between the two outcrops. It seems to have been over this bed of black, 

 shaly limestone that most of the ash was originally deposited, the Croghan 

 Hill ash and that in the neighbourhood of Boston being at one time a con- 

 tinuous bed. The small anticlinal fold, just north of the main vent, was 

 probably caused by the disturbances that took place at the close of the 

 carboniferous period when the Hereynian folds were formed. From the 

 summit of this anticline, the ash and the underlying bed of black, shaly 

 limestone have been worn away, separating the ash into two parts, and 

 exposing the upper beds of the Lower limestone. 



\fish 



Pw^dl Basalt I ^ Middle Limestone (Calp) ITl * tI Lower Limestone. 



Fig. 1. — Diagrammatic Section across Croghan Hill. 



No very conclusive evidence of the interstratification of the ash and 

 limestone was observed, but a few instances occur which throw some light 

 on the age of the volcanic eruptions. As noted by Sir A. Geikie, 1 one of 

 these can be seen near Gorteen on the south-west side of the hill. Here 

 there is a band of limestone only a few feet thick, with ash above and 

 below it, and, although the junction between the two is well defined, no 

 trace of any volcanic material can be seen in the limestone. Pieces of chert 

 and limestone are, however, of common occurrence in the ash. Another 

 instance occurs at the most easterly neck, just north of Coolmount House, 

 where the limestone, much undulating, appears to dip below the ash. On 

 the northern side of this neck the limestone is seen, again rapidly undulating, 

 and it finally plunges southwards beneath the ash on the hill. The ash, 

 which here overlies the limestone, shows a series of divisional planes 

 suggestive of bedding, which dip south parallel with the limestone. About 

 two hundred yards west of this point a well has been sunk to a depth of 

 twelve feet. This passes through about six feet of dark limestone with bands 

 of shale and chert. This is seen to be underlain by a bluish, grey clayey 

 material which is highly calcareous, and when examined had all the 

 appearance of weathered ash, as it consisted of small vesicular fragments of 

 pale grey pumice lying in a clayey matrix. This material appears to be ash 

 weathered in situ, which has been decomposed by the mfiltering of water 

 through the over-lying limestone. This limestone is evidently a continuation 

 of the dark, shaly beds which occur a little further to the east, where they 



1 Op. tit., p. 39. 



