CHAP. XXX 
THE LIMERICK BASIN 
43 
more complete for the occurrence of a long succession of volcanic eruptions 
during a definite period of geological time. The officers of the Siuyey 
showed that two epochs of activity during the older part of the Uu- 
boniferous period were each marked by a group of tufts and lavas while 
the interval of quiescence between them is represented by a thousand leer 
of limestone. The same observers likewise mapped outside the volcanic 
ring a number of eruptive bosses, which they regarded as probably marking 
some of the actual vents of that time. , 
The lower volcanic group, which forms a complete ring round t le 
Upper Limestones of the Limerick basin, is estimated to reach a thickness ot 
1000 feet in some parts of its course. 1 Its base appears to coinci e 
generally with the upward termination of the Lower Limestone group o 
this district, though here and there small patches of volcanic rocks in that 
group have been regarded as interstratified and contemporaneous bands. 
It consists of a series of lavas and tuffs, the alternations and rapid incoming 
and dying out of which were well made out by the Geological Survey. 
Tuffs — The base of the volcanic series is generally formed by a band o 
tuff sometimes as much as 350 feet thick, 3 which maybe traced nearly 
continuously round the basin as well as in detached outliers even as lai as 
Carrigogunnel overlooking the alluvial plain of the Shannon. The manne 
in which the bottom of this tuff is interstratified with the limestone be 
it may be instructively examined in many quarries around the town 
Limerick. Striking evidence is there supplied that the first eruptions were 
comparatively feeble and spasmodic, and were separated y m 
longer and shorter duration, during which the limestone wit i l ■ « 
ary organisms was deposited, little . or no volcanic detritus falling at tha 
time. Yet even in some of the limestones the microscope reveal fine 
broken needles of felspar, representing doubtless the finest ejected dus . 
As an illustration of the way in which the volcanic and organic detut 
alternated over the sea-fioor, the following section from a quarry m_ 
townland of Loch Gur on the southern side of the basin is here given . 
• S"^h^Lacte rS that rather seem to place 
materials of the district, and therefore not necessarily eailiei than le t horizon n 
The boundary line of the volcanic series is not consistently followed along the same homo o 
the Survey maps. Thus to the east of Caherconlish, a strip of the Upper Limestone is insem 
telow the base of the tuffs for a distance of about four miles. Unless a < 1 g .J. would 
teen in some places taken for the boundary between the two groups o (lis trict until 
appear that the eruptions had not extended over the north and 
some time after the deposition ot the Upper Limestone had begun. mv-tant it ia some- 
two limestone groups is taken at a set of chert-bands, but as these are not constant it 
times difficult to draw a satisfactory line of division. 
• — - *>» v'vx’ 1 ' 
indebted to the examination of them made for me by my Survey colleague, 
0 Explanation of Sheet 154, pp. 21, 22. 
