CHAP. XIV 
LLANDEILO AND BALA ERUPTIONS OF IRELAND 
247 
lenticular bands so common in Caernarvonshire. In like manner the nodular 
structure, though not entirely absent, is rare. 1 . -i i • roofimfinn 
Until these felsites have been sulyectcd to more detailed investigat , 
little can be said as to their petrography, and as to the points of 
or difference between them and those of other Lower 
the United Kingdom. An important step, however, in 
taken by Ur. Hatch, who studied tliein on the ground, in the laboiatorj 
and with the microscope. He found that some of them were «oda-fe sites 
or keratophyres (with albite as their felspar), that others were 
(with orthoclase as their felspar), while a third group contained both soda 
and potash, the last-named greatly preponderating.- The e.xistence of soda- 
felsites had not been previously detected among British volcanic vS, a 
it remains to be seen how far they may occur in the large and somewhat 
varied group of rocks combined under the general term elsites. Ui 
Hatch believed that these rocks probably graduate into the noinial 
orthoclase felsites; but it has not yet been possible to test 
the ground, nor to ascertain whether there is any essential difference betiveen 
the mode of occurrence of the two types. , , , . 
Besides the more abundant felsites, occasional bands of andesite have 
been detected. Various other eruptive rocks occur, 
all cases intrusive. Such are qiiartz-mica-diorites, quartz-diorites, au,,ite 
diorites or proterobases, dolerites, gabbros, diabases and 
I have said that the chief theatre of eruption lay towards the south-west 
end of the volcanic belt of the south-east of Ireland. Ihe coast-line 
County Waterford, from Tramore westward to Ballyvoyle Heac a c is anee 
Sy Mteen „,iL_prese„.., perhaps, the .nost wonCerh.l ser.es ot s«,.o„ 
Of volcanic vents within the British Islands. No one coming from the 
inland is prepared for either the striking character of the 
the extraordinary geological structure there presented, for the country s 
on the whole, rather featureless, and much of it is smootlmd over 
obscured by a covering of drift, through which occasional knobs of the 
harder felsites protrude. The cliffs for mile after mile from 100 
to 150 or 200 feet in height, and present naked vertical walls of rock, 
trenched by occasional gullies, through which a descent I’® 
the beach. Throughout the whole distance agglomerates and 
each other in bewildering confusion, varied here and there ^ 
calation of Lower Silurian shales and limestones involved and 
igneous rocks. Hardly any bedded volcanic material is to be 
one end to the other. The sea has laid bare a succession of volmiic ven s 
placed so close to each other that it will be difficult or impossible to separate 
them out. A careful study and detailed inapping of this marvellous coast- 
section, however, is a task well worthy of the labour of any one desirous 
1 111 Waterford nodular felsites occur with concretions varying from the size of a pea to seveial 
inches in diameter. Explanation to Sheets 167, 168, I18 and 1/9, p. ^ . 
- Explanation of Sheets 138, 139, p. 49 ; and (leol. Mo.g. 1889, p. o4o. 
Guide io Insh Rock-CollcctionSj pp. 34, 35. 
