154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ash " is a dull green tuff full of fragments of felspar (chiefly plagio- 

 clase) and pieces of dark andesitic lavas. Another example may 

 be found to the west of the Metal Man, near Traraore, where the 

 tuff is full of fragments of felspar and shale cemented in a greenish- 

 5'ellow material which may be palagonite. 



The felsites of the south-east of Ireland form by much the 

 largest proportion of the whole volcanic series. They occur as 

 lenticular sheets from a few feet to several hundred feet in thick- 

 ness, and occasionally traceable for some miles. On the whole, 

 they are compact dull grey rocks, weathering with a white crust. 

 A geologist familiar with the contemporary lavas of North Wales 

 cannot fail to be struck with the absence of the coarse flow-' 

 structure so often characteristic of the felsites in that region. This 

 structure, indeed, is not entirely absent from the Irish rocks, but it 

 occurs, so far, at least, as I have seen, rather as a fine streakiness 

 than in the bold lenticular bands so common in Caernarvonshire. 

 In like manner the nodular structure, though not entirely absent, 

 is rare *. 



Until these felsites have been subjected to more detailed investi- 

 gation, little can be said as to their petrography, and as to the points 

 of resemblance or difference between them and those of other Lower- 

 Silurian districts in the United Kingdom. An important step, 

 however, in this direction has been taken by Dr. Hatch, who three 

 years ago studied them on the ground, in the laboratory, and with 

 the microscope. He found that some of them were soda-felsites or 

 keratophyres (with albite as their felspar), that others were potash- 

 felsites (with orthoclase as their felspar), while a third group con- 

 tained both soda and potash, the last-named greatly prepon- 

 derating t. The existence of soda-felsites had not been previously 

 detected among British volcanic rocks, and it remains to be seen 

 how far they may occur in the large and somewhat varied group of 

 rocks combined under the general term "felsites." Dr. Hatch 

 believes that these rocks probably graduate into the normal or 

 orthoclase felsites ; but it has not yet been possible to test this 

 view on the ground, nor to ascertain whether there is any essential 

 difference between the mode of occurrence of the two types. 



I have said that the chief theatre of eruption lay towards the 



"* In Watei'ford nodular felsites occur with concretions varying from tlie size 

 ol a pea to several inches in diameter. Explanation to Slieets 167, 16S, 178, 

 and 179, p. 11. 



t Explanation of Sheets 138, 139, p. 49, and Geol. Mag. 1889, p. 545. 



