146 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



west of Ireland.^ Nowhere, indeed, is the evidence more complete 

 for the occurrence of a long succession of volcanic eruptions during 

 a definite period of geological time. The officers of the Survey 

 showed that there were here two chief periods of activity during the 

 older part of the Carboniferous period, each of them being marked 

 by a group of tuflfs and lavas, while the interval of quiescence 

 between them is represented by a thousand feet of limestone. During 

 a visit to this most instructive region, in company with my col- 

 leagues Messrs. M'^Henry and Watts, I was interested to find that it 

 afi'ords so admirable an illustration of the puy-type of eruption. At 

 the base of this volcanic series to the east of Limerick, there is 

 striking evidence that the first eruptions were spasmodic, with 

 intervals of longer and shorter duration, during which the compact 

 black limestone with its fragmentary organisms was deposited, 

 little or no volcanic detritus falling at that time. Yet even in some 

 of the limestones the microscope reveals fine broken needles of 

 felspar, representing doubtless the first ejected materials.^ As we 

 trace the strata upwards they pass into the ordinary tufi' of the 

 lower volcanic group. This tuff' abounds in porphyrite (andesite) 

 fragments, with a few felsitic rocks, enclosed in an opaque ground- 

 mass, through which are scattered broken felspars and occasional 

 vesicular lapilli. The tuff's of the upper group are readily distin- 

 guishable from those of the lower, even in the field. They are 

 found, when examined microscopically, to contain abundant vesicular 

 lapilli of palagonite-tuff* with fragments of various amygdaloids, 

 and bombs of a basic pumice. 



The lavas occur in numerous sheets, sometimes separated by beds 

 and even thin partings of tuff and volcanic conglomerate. This 

 alternation of lava-streams and showers of ashes is well displayed 

 in the upper group above Nicker.-'^ Some of the flows are porphy- 

 rites, showing the characteristic andesitic base of minute felspar- 



1 See especially Explanations of Sheets 144 & 154, Geol. Surv. Ireland (1860, 

 1861). The geology of the district had been previously noticed by earlier 

 observers, to whose writings reference is made on p. 26 of the Explanation of 

 Sheet 144. See also Jas. Apjohn, Journ. Greol. Soc. Dublin, vol. i. (1832) p. 24 ; 

 Edw. Hull, Geol. Mag. for 1874, p. 205. The microscopic structure of some of 

 the Limerick volcanic rocks has been described by Mr. Allport, Quart. Journ. 

 Geol. Soc. vol. XXX. (1874) p. 552, and by Prof. Hull, Geol. Mag. for 1873, p. 153. 



2 The details of the microscopic structure of the Limerick volcanic rocks here 

 given are taken from notes of a preliminary examination undertaken for me by 

 Mr. W. W. Watts. But see the papers cited in the previous note. 



3 Explanation of Sheet 144, Geol. Surv. Ireland, pp. 26-34. 



