CHAP. XIV 
LLANDEILO AND BALA ERUPTIONS OF IRELAND 
241 
diabases, these siliceous bands l^ear the closest resemblance to tliose ot 
Arenig age in Scotland. Hut no recognizable Hadiolaria liave jet been 
detected in them. 
Besides the more basic lavas, there occur also, but less abundantly, platy 
felsitic rocks which have suffered much from shearing, and consequently 
have acqitired a fissile slaty structure. 
Tlie agglomerates are made up of angular, subangular and rounded 
fragments imbedded in a matrix of similar composition. This matrix has 
in places become quite schistose, and then closely resembles some parts of 
the “ green schists ” of the Scottish Highlands. Of the inclosed stones 
the great majority consist of various felsites, which, weathering with a thick 
white opaque crust, are internally close-grained, dull-grey or even black, some- 
times showing flow-structure, and of all sizes up to eight inches in diameter 
or more. There are also fragments of the basic lavas, and likewise pieces 
of chert and jasper. On many of the rocky hummocks no distinct bedding 
can he made out in the agglomerate, but in others the rock is tolerably well 
stratified. 
The tuffs are line silky schistose rocks, and seem to liave been largely 
derived from basic lavas. They have suffered more than any of the other 
rocks from mechanical deformation, for they pass into green chloritic schists. 
Some portions of them are not unlike the slaty tuffs ot Blyn l adain in 
Caern ar von shire. 
Accompanying the fragmental volcanic rocks, some ordinary sedimentary 
intercalatiorrs are to be fourrd — red shales and pebbly qrrartzites, that 
seerrr to have escaped nruch crushiirg. The true order of successiorr 
itr the volcanic series has irot yet beerr deterrniired. But appar’errtly above 
this series come some dark shales, sirclr as rrright yield gruptolites, pale grits 
and occasional limestones. 
Later tharr the lavas and the pyroclastic material are variorrs irrtrrrsive 
masses, which in bands arrd bosses fontr rrurrrerous craggy hills throughorrt 
the area. So far as I have been able to observe, these rocks include two 
groups. Of these the older consists of basic iirjeetions, such as gabbros 
and allied rocks, .sorrre of which remind rrre of the so-called “ hyperstherre- 
rock ” of Lendalfoot, irr Ayrshire. The coarser varieties, as at Carrickmore 
or Tennon rock, are sometimes traversed by fine-grained veins from an inch 
to several feet in breadth. I’ortions of the slaggy diabases may he observed 
inclosed in these intrusive masses. The younger group is of more acid 
composition (granite, quartz-porphyry, etc.), and sends veins into the older. 
ii. Erv/ptions of lAunclcilo and Bcda Age 
Into the east of Ireland the Lower Silurian rocks are prolonged from 
Scotland, from the Lake District and from Wales. Though greatly concealed 
under younger formations across the breadth of the island, and occasionally 
interrupted by what are regarded as older strata of Cambrian age, they 
nevertheless occupy by much the larger part of the maritime counties 
VOL. I ® 
