346 
VOLCANOES OF THE LOWER OLD RED SANDSTONE 
BOOK V 
intrusion of a series of bosses and dykes of basic rocks (diabases) which 
traverse the sills. 
The Killakney District 
In the south of Ireland the Upper Silurian strata are followed upwards 
conforinably by the great series of red sandstones and conglomerates known 
as the “ Dingle Beds.” Lithologically these rocks present the closest 
resemblance to the Lower Old Bed Sandstone of Central Scotland. They 
occupy a similar stratigraphicid position, and though they have not yielded 
any pahuontological data for comparison, there can, I think, be no hesitation 
in classing them with the Scottish Lower Old Eed Sandstone, and regarding 
them as having been deposited under similar geographical conditions. They 
offer one feature of special interest for the piupose of the present inquiry, 
since they contain a well-marked group of contemporaneous volcanic rocks, 
including nodular felsites, like those so characteristic of the Silurian period. 
The area where tliis remote and isolated volcanic group is best developed 
forms a range of high rugged ground along the northern front of the hills 
that stretch eastward from tlie Lakes of Killarney. Their general distribu- 
tion is shown on Sheets 184 and 185 of the Geological Survey of Ireland;^ 
tliough I may again remailc that petrography has made great strides during 
the thirty years and more that have passed since these maps and their 
accompanying Memoirs were published, and that, were the district to be 
surveyed now, probably a considerable tract of ground coloured as ash would 
be marked as felsite. At the same time the existence of both these rocks 
here cannot be gainsaid. 
The felsite was long ago brought into notice by Dr. Haughtou, who 
published an analysis of it.’-^ It is also referred to by Mr. Teall for its 
spheruhtic structure.® Seen on the ground it appears as a pale greenish- 
grey close-grained rock, sometimes exhibiting flow-structure in a remarkabh- 
clear manner, the laminae of devitrification following each other in wavv 
lines, sometimes twisted and delicately puckered or frilled, as in some 
schists. Portions of the rock are strongly nodular, the nodules varying in 
size from less than a pea to that of a hen’s egg. 
The close resemblance of this rock to many of the Lower Silurian 
nodular felsites of Wales cannot but strike the geologist. It presents 
analogies also to the Upper Silurian felsites of Dingle. But its chief 
interest arises from the geological horizon on which it occurs. Lying in 
the so-called “Dingle -Beds,” which may be regarded as the equivalents 
of the Lower Old Eed Sandstone of England and Scotland, it is, so far 
as my observations go, the only example of such a nodular felsite of 
later date than the Silurian period. We recognize in it a survival, as it 
were, of the peculiar Silurian type of acid lava, the last preceding eruption 
1 See the Memoir (by .1. B. Jukes and G. V. Du Koyer) on Sheet 184, p. 15. Other volcanic 
rocks have been mapped at Valentia Harbour in the Dingle Beds, but these I have not had an 
opportunity of personally examining. 
^ Trans. Roy. Irish Acad. vol. xxiii. (1859), p. 615. » British Petrography, p. 349. 
