Vol. 51.] THE COTJNTKX ABOUND FISHGUARD. 177 



(d) Chemical Composition of the Volcanic Rocks. 



The highly acid composition of the volcanic rocks in this area is 

 apparent both in the field and under the microscope. But chemical 

 analysis brings out several points of interest. 1 The dark bluish- 

 grey rock [76] from Goodwick (described on p. 162), with a specific 

 gravity of 2-60, has the following composition (A). 



A. B. 



SiO, 68-01 7206 



Al 2 6 3 1748 14-87 



Fe 2 3 041 1-33 



K 2 708 5-90 



Na 2 5-77 4-11 



MgO 046 1-26 



CaO trace trace 



H 9 0-73 026 



99-94 99-79 



Another felsite [203] from Cam Gelli (described on p. 163) has a 

 higher percentage of silica. The specific gravity is 2'70. See 

 analysis (B) above. 



These two rocks have a remarkably high percentage of alumina, 

 potash, and soda combined. In the first these three components 

 make up 30*33 per cent, of the whole, and in the second 24-88 per 

 cent, of the whole. Such a proportion militates against classing 

 them with the rhyolites proper, and leads us to compare them with 

 the quartz-keratophyres of the Continent. Keratophyres were first 

 described by Giimbel in association with the Cambrian beds of the 

 Fichtelgebirge, 2 and since then our knowledge of these rocks has 

 been considerably increased by the researches of Lossen in the 

 Harz. 3 Tbe high percentage of alkalies in the keratophyres at 

 once recalls the leucite- and nepheline-bearing rocks, but these 

 minerals are nevertheless absent. There is considerable variability 

 in the amount and relative proportions of the alkali-felspars, in the 

 percentage of silica, and in the texture of the rocks. Thus some 

 are compact and hornstone-like in appearance, and Bosenbusch 

 states that those with a microfelsitic facies have as yet been 

 scarcely noticed in Europe. 4 Howitt describes some of Palaeozoic 

 age in Gippsland, 5 associated with intrusive masses. Bosenbusch 

 states that some of the "Welsh rocks, which are usually termed quartz- 

 felsites, would be more correctly named ' quartz-keratophyres ' ; 

 and he instances the Llyn Padarn rocks and the nodular felsites of 

 Conway Falls, Digoed, Cwmyglo, described by Bonney, Cole, and 

 others. The potash-soda and soda-felsites of Co. Wicklow described 



1 The analyses have been made for me by Mr. F. E. Tadman, of Queens' 

 College, Cambridge, in the University Chemical Laboratory. 



J ' Die palaolithischen Erupt ivgesteine des Fichtelgebirges,' Munich, 1874, 

 p. 45. 



3 Zeitschr. Deutsch. geol. Gesellsch. vol. xxxiv. (1882) pp. 199, 455, and 

 vol. xxxv. (1883) p. 215. 



1 ' Mikroskopische Physiographie,' 2nd ed. vol. i. (1885) p. 440. 



1 'The Rocks of Noyang,' Trans. Roy. Soc. Victoria, vol. xx. (1883) p. 19. 



