244 
THE SILURIAN VOLCANOES 
BOOK IV 
rounded pebbles, give place to a massive bed of agglomerate, the enclosed 
blocks of which are always of one species of felstoiie, sometimes measuring 
10x12x1 8 inches, and not always rounded.” South of Carrickatee Lough, 
and a few miles farther to the south-west, near Lackan Bridge, considerable 
exposures of these rocks occur. One crag in particular di, splays a thickness 
of more than 7 0 feet of “ tough flaky breccias,” “ thick agglomerates with 
small and large blocks of felstone,” and “ thin beds of fine pale green compact 
grit without pebbles, and a few flags.” “ One of the flaky beds cont a ins 
numerous white worn crystals of felspar ” ; “ the imbedded blocks of felstone 
are of the usual kind — pale compact matrix showing dark oblong patches, 
vesicular and amygdaloidal, the cavities being filled with chlorite.” 
Further south a more extensive area of igneous rocks has been mapped 
on the borders of Louth and Meath, where, according to the Geological 
Survey map, a group of lavas and tufl's extends for about twelve miles near 
Slane.' Other bands of “ ash ” and “ felstone ” liave been mapped in the 
Silurian area south of Drogheda. Thus at Hilltown, west from the race- 
course, a “ bluish crystalline felstone, showing in places lines of viscous 
flow,” is stated to be overlain by “ indurated felspathic ash and tuff, felstone, 
and indurated shale ” in alternating beds.“ On a recent visit to this locality 
I found that the “ porcellanite or indurated shale ” is a greenish-grey chert, 
full of Eadiolaria and finely-diffused volcanic dust. This association of 
radiolarian chert with contemporaneous volcanic activity is of much interest, 
as showing the extension of the same physical conditions of the Lower 
Silurian sea from Scotland into Ireland. The Lower Llandeilo age of the 
volcanic intercalations in County Meath is further indicated by the occur- 
rence of Didymoffraptus Ahirchisoni in grey shales in the same neighbour- 
hood with the radiolarian cherts. In the Lower Silurian district of 
Balbriggan numerous intrusive bosses and sills have been mapped by the 
Geological Survey. I have found, however, that among these rocks there 
occur bands of volcanic breccia, containing abundant angular fragments of a 
minutely-vesicular pumice, and also that some of the diabase-masses display 
the pillow-structure and amygdaloidal texture. Hence, though most of the 
igneous rocks are no doubt intrusive, they appear to include lavas and tuffs 
of Bala age. 
When the numerous Silurian cores of the mountain -groups in the 
interior of Ireland shall have been searched for traces of contemporaneous 
volcanic action, it is not improbable tliat these will be found. One of the 
smaller Silurian inliers which diversify the great Carboniferous plain, that 
' Bdd. Sheets 81 anil 91. These rocks are chiefly augitic andesites, a few are basalts, and 
some seem related to felstones. Probably many of them are intrusive sills of uncertain age. The 
•• ashes ” contain fragments of felsite and porphyrite often of considerable size {Guide to Irish 
Eock-ColUctioii, )i. 36). 
“ Ibid, Sheets 91 and 92 and Explanation to these Sheets (1871), p. 10 ; Guide to Irish Rock- 
CoUcctwn,, p. 36. Some of these lavas are andesites, others are fetsites. Mr. M ‘Henry has contended 
that certain “ashes” and “agglomerates,” particularly those exposed on the coast at Portraine, 
opposite Lanibay Island, are “ crush -conglomerates ” due to terrestrial disturbances, which have 
affected both intrusive igneous rooks and the sedimentary series into which these have been injected. 
