Haigh — Carboniferous Volcanoes of Philipstotvn, King's Co. 19 



Croghan Hill in the projecting crags. The fragmentary material in the ash 

 varies from the size of a walnut to dust so fine that it can only be observed 

 with the aid of a microscope. It consists of angular and subangular pieces of 

 pale grey vesicular pumice; the vesicles are often filled with calcite and 

 a green chloritic mineral. In describing this ash Sir A. Geikie 1 says : 

 " I am not aware of any other necks so homogeneously filled up with one 

 type of pyroclastic material, and certainly there is no other example known 

 in the British Isles of so large and uniform a mass of fragmentary pumice." 

 The ash is very calcareous and in some parts might be described as an ashy 

 limestone. It is hard to account for the presence of so much calcareous 

 material in the ash, unless, as Sir A. Geikie suggests, the vents were opened 

 on the floor of the carboniferous sea, when fine calcareous silt would find its 

 way down into the interstices of the ash, and into the pores of the pumice. 

 Although percolating water containing carbonate of lime in solution may 

 have added to this, it does not seem likely that this process would in itself 

 be adequate to so thoroughly saturate the ash. 



The limestone directly underlying the ash, in the few sections visible, is a 

 dark impure shaly rock, very similar lithologically to the "Calp" or "Middle 

 limestone " so well developed in the neighbourhood of Dublin. The ash 

 contains fragments of the limestone and chert embedded in it, and in a 

 few instances pieces of basalt were found enclosed in the ash. Some of 

 these limestone and chert fragments were distinctly angular, and had 

 evidently been blown out of the vent when it was first opened, and thus 

 became embedded in the accompanying ash. Other pieces are quite rounded 

 and have all the appearance of water-worn pebbles and boulders. Similar 

 rolled pebbles have been found in the volcanic area near Limerick, and in 

 describing these Mr. J. E. Kilroe 2 says : " The more natural way of account- 

 ing for the circumstance is to attribute them to the action of waves on an 

 exposed mass of basic lava, the results of attrition being then distributed 

 over the neighbouring sea-floor, where ash was accumulating." The majority 

 of the pebbles found in the Croghan Hill ash are not basalt, but chert and 

 limestone, particularly chert. 



If the ash was deposited directly on the dark, shaly limestone while the 

 latter was accumulating on the sea-floor, the sea must have been a shallow 

 one, and the vent, with its surrounding limestone, was probably a small 

 volcanic island, against which the waves of this shallow sea beat, breaking 

 down the limestone shore and finally eroding away the softer limestone, 



1 "Ancient Volcanoes of Great Britain," vol. ii, 1897, p. 39. 



1 Memoirs Geol. Surv. Ireland, " Geology of the Country round Limerick," 1907, p. 39. 



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