CHAP. XIV LLANDEILO AND BALA ERUPTIONS OF IRELAND 
249 
they have been tossed about, crushed and invaded by dykes and veins of 
^^^^'sut certain other intercalated strips of stratified rock.s present a special 
interest, for they bring before us examples of volcanic ashes that gathered 
on the sea-fioor, but which were disrupted by later explosions. Thus, at 
Knockmahon headland, well-bedded felspathic grits and ashy shales occur, 
thrown in among the general mass of eruptive material. 4 ® ^ 
remarked, it is difficult or impossible to fix the horizons of the stiatiti 
patches that are involved among the igneous ejections of this «iast-section 
Lve where they contain rccogiiizalde fossils, but the iiitercalatnm of ti 
bedded tuffs among them is a proof that volcanic action had been m 
operation there long before the outbreak of the vents which are now laid 
bare along the cliffs. . , , 
In the south-east of Ireland there is the usual association of acid and 
basic sills with the evidence of a superficial outpouring of lavas and as les. 
But these intrusive masses play a much less imposing part t lau in ^ 
They may be regarded, indeed, as bearing somewhat the same proportion to 
the comparatively feeble display of extrusive rocks 111 this region tliat the 
abundant and massive sheets of IVIerionethshire and Caernarvonshiie do to^ 
the enormous piles of lavas and tuffs which overlie them. 
Anion- the acid intrusive sheets tlie most conspicuous are those maiiped 
by the Survey as “ elvans.” These rocks, as they occur in W icklow and 
W^exford have been examined by Dr. Hatch, who finds them to be niicro- 
-ranitic in structure, occasionally exhibiting micropegmatitic or granophjric 
modifications.^ The true stratigraphical relations of these -‘' 1 - 
yet been adequately investigated. Those of them which occur on 1 e fianks 
of the great granite ridge are not improbably connected wi^ that mass, a 
if so are much younger than the Lower Silurian volcanoes. 
The basic sills, or “greenstones,” consist largely of diabase, hcquent j 
altered into epidiorite ; they include also varieties of diorite. That thej 
were intruded before the plication and cleavage of the rocks among vhicl 
they lie is well shown by their crushed and sheared margins where they aie 
in thick mast, and by theiv cleaved and ahaost ecliistee co„d.t, on where 
they are thinner. The intense compression and crushing to vbnh t } 
hairLu enbjeced are well shown by the state of the, r c^ponen 
minerals, and notably by the paramorphism of the original au^ite 
hornblende. •, „„ 
The scarcity of dykes associated with S.lnrnm volcamc “ 
noticeable in the sonth-east ot Ireland as it is ... W..les have el se, v c 
a considereble m.n.l*r, in.leed, bnt they arc oonhned to the line of oh re . s 
on the Wate.-ford coast, and, bnt tor the clear cliff-seotlons c.t by the 
sea, they would certainly have escaped observation, tor the} make n 
’ Exiylaiiation of Sheets 138 anil 139, p. 53. _ than the 
- The Leinste.v granite is certainly later than the Low ei luuan locv , f erranite 
Carboniferous rocks of the .south-east of Ireland. It may belong ^ ^ 
protrusion during the Old Red Sandstone period. ^ ^ ’ 
