38 
BOOK VI 
THE CARBONIFEROUS VOLCANOES 
though the admission is made that “ from the scanty exposures of the rocks 
and the total absence of any connected section, it has been found impossible 
to arrive at any definite conclusion as to the relations existing between 
these traps and ashes with regard to each other or to the surrounding 
limestone. 1 ° 
In the course of a brief visit to this locality I did not succeed in 
obtaining any certain proof of the age of the igneous rocks, but I found 
ieir structures to be more varied and interesting than would be inferred 
from the way in which they have been mapped, and I came to the con- 
clusion that the strong balance of probability was in favour of regarding 
them as of the age of the Carboniferous Limestone. 
The first and most important fact to be announced regarding the 
district is that it includes a group of volcanic necks which rise through the 
Carboniferous Limestones. The chief of these forms Croghan Hill. It is 
nearly circular in ground-plan, and measures about 4000 feet in diameter 
Fig. 192.— Croghau Hill, King’s County, from S.S.W. 
from the limestone on one side to that on the other. It rises with steen 
grassy slopes out of the plain, the naked rock projecting here and there in 
crags and low cliffs. Its general outward resemblance to the Carboniferous 
of Scotland strikes the eye of the geologist as he 
approaches it 
necks 
(Fig. 192). 
But Croghan Hill, though the chief, is not the only vent of the district 
It forms the centre round which a group of subsidiary vents has been 
opened These form smaller and lower eminences, the most distant beiim 
one and a half miles E.S.E. from the summit of Croghan Hill, and measurin' 
approximately 1200 feet m its longest and 800 feet in its shortest diameter 
That the igneous materials of these necks really break through the 
imestones may be clearly seen in several sections. Thus by the roadside 
at G oi teen, on the south-western side of Croghan Hill, the limestones have 
( ! n U hl ° Wn mt ? a , ln S hl y inclined position, dipping towards the east at 
°i moie ’ ancl their truncated ends abut against the side of the neck. 
Again, on the eastern side of the same hill the limestones have been much 
Sheets 98, g^loS^ndlO^by K J.^Sfand I O'Kelty (1865)! ppf ^g lanation to accompany 
