[eoit} 
LXIV.—ON CHERT IN THE LIMESTONE OF KNOCKBEG, 
COUNTY FERMANAGH, sy THOMAS PLUNKETT. 
[Read June 21st, 1880. | 
CHERT occurs in a variety of forms in nearly all the limestone 
hills from Florence Court to the bold headland known as Ben 
Bulbin, a few miles north of Sligo, including Knocknarea, to the 
west of that town. 
Occasionally it may be found in boss-like masses studded over 
the surface of the limestone. It sometimes assumes the nodular 
form, but it is most commonly found in thin bands or layers 
intercalated with the limestone beds. 
Knockbeg, as its name implies, is a little hill or elevated 
rocky surface; it is composed of limestone containing nodules 
and bands of chert. There is a strip of ground at the south 
margin of the flat hill top, about twenty perches long, and from 
two to four perches broad, in which are found vertical sheets of 
chert passing up at right angles through the almost horizontal 
limestone strata, and projecting from two to four feet above the 
surface, resembling huge flags set on edge. At first sight one 
might imagine that this strip of ground had been a pagan 
cemetery, containing a number of mutilated “ giants’ graves.” 
The rocky surface having been exposed to the weathering of 
ages the limestone has yielded more freely to atmospheric 
erosion than the sheets of chert, leaving the latter standing up 
in bold relief. These projecting ribs of chert generally run in 
a north-westerly direction, and are from three to twelve feet in 
length ; a few, however, run due west; they are not continuous 
in regular lines but occur somewhat as represented in the 
diagrammatic sketch on the next page. 
