^8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



developed forms a range of high rugged ground along the northern 

 front of the hills that stretch eastward from the Lakes of Killarney. 

 Their general distribution is shown on Sheets 184 and 185 of the 

 Geological Survey of Ireland ; ^ though I may again remark that 

 petrography has made great strides during the thirty years and more 

 that have passed since these maps and their accompanying Memoirs 

 were published, and that, were the district to be surveyed now, 

 probably a considerable tract of ground coloured as ash would be 

 marked as felsite. At the same time the existence of both these 

 rocks here cannot be gainsaid. 



The felsite was long ago brought into notice by Dr. Haughton, 

 who published an analysis of it.^ It is also referred to by Mr. Teall 

 for its spherulitic structure.^ Seen on the ground it appears as a 

 pale greenish-grey close-grained rock, sometimes exhibiting flow- 

 structure in a remarkably clear manner, the laminae of devitrifica- 

 tion following each other in wavy lines, sometimes twisted and 

 delicately puckered or frilled, as in some schists. Portions of the 

 rock are strongly nodular, the nodules varying in size from less 

 than a pea to that of a hen's egg. 



The close resemblance of this rock to many of the Lower Silu- 

 rian nodular felsites of Wales cannot but strike the geologist. 

 It presents analogies also to the Upper Silurian felsites of 

 Dingle. But its chief interest arises from the geological horizon on 

 which it occurs. Lying in the so-called ' Dingle Beds,' which, as 

 I have said, may be regarded as the equivalents of the Lower 

 Old Bed Sandstone of England and Scotland, it is, so far as 

 my observations go, the only example of such a nodular felsite or 

 rhyolite of later date than the Silurian period. We recognize in it 

 a survival as it were of the peculiar Silurian type of acid lava, the 

 last preceding eruption of which took place not many miles to the 

 west, in the Dingle promontory. But elsewhere this type did not 

 survive the end of the Silurian period. 



The detrital rocks accompanying the felsite in the district east of 

 Killarney vary from such close-grained felsitic material as cannot 

 readily be distinguished from the felsite itself to unmistakable 

 felsitic breccias. Even in the finest parts of them occasional 



^ See the Memoir (by J. B. Jukes and Gr.^V. Du l^oyer) on Sheet 184, p. 15. 

 Other volcanic rocks have been mapped at Valentia Harbour in the Dingle 

 Beds, but these I have not had an opportunity of personally examining. 



2 Trans. Eoy. Irish Acad. vol. xxiii. (1859) p. 615. 



3 ' British Petrography,' p. 349. 



