246 
THE SILURIAN VOLCANOES 
KOOK IV 
an inch in thickness, lie among the shales. In some of the tuffs the lapilli 
are fragments ol trachytic or andesitic rocks. 
A striking example of rapid alternations of pyroclastic material with 
ordinary sediment lies far to the south in County Waterford, close to Dun- 
hill Bridge, where a group of fine volcanic breccias and grits has been 
laid bare by quarrying.^ These strata consist of coarser and finer 
detritus, enclosing angular fragments of felsites and grey and black shale. 
The felsite-lapilli vary in texture, some of them presenting beautiful flow- 
structure. The stones are stuck at random through each bed, the largest 
being often at the bottom. The beds of breccia vary from a few inches to 
a foot or more in tliickness. There can, I think, be little doubt that each 
of these breccia-bands points to a single volcanic explosion, whereby felsitic 
fragments were thrown out, mingled with pieces of the Silurian strata throu^vh 
which the vents were drilled. In a vertical thickness of some fifty feet "’of 
rock there must thus be a record of ten or twelve such explosions. 
Nearer the active vents the fragmental deposits become, as usual, coarser 
and thicker. But I have not oliserved any thick masses of tuff like those 
of North Wales. So far as my examination has gone, the tuffs are mainly 
felsitic. The so-called “ greenstone-ash ” of the Survey maps is certainly in 
many cases not a true tuff. This term was proposed by Jukes for certain 
apple-green to olive-brown flaky fissile rocks only found " in association with 
masses of greenstone.”- Some years ago I had occasion to make a series of 
traverses in Wicklow and Wexford, and then convinced myself that in that 
part of the country the “ gi'eenstone-ashes ” were probably crushed bauds of 
basic sills. Dr. Hatch has proved tliis to be their origin from a series 
of microscopic slides prepared from specimens collected by himself on the 
ground.^ In other cases the “greenstone-ashes” seem to he excessively- 
cleaved or sheared felsites, which have acquired a soapy feel and a dull o-reen 
colour ; but they also do include true tuffs. Thus, in one instance, at Bally- 
voyle cross-roads, in the south of County Waterford, a “ greenstone-ash ” 
IS a dull green tuff full of fragments of felspar (chiefly plagioclase) and 
pieces of dark andesitic lavas. Another example may lie found to the west 
of the Metal Man, near Tramore, where the tuff is full of fragments of 
lelspar and shale cemented in a greenish-yellow 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 thickness, 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 liave seen, rather as a fine streakiness than in the bold 
‘ See Exi.Iaiiation of Slieets 167, 168, 178 and 179, Geol. Suvv. Ireland, p. 56 
Explanation of Sheet, 129, 130, p. 13 (1869). « Explanation of Sheet, 138, 139. 
