On Chert in the Limestone of Knockbeg, Co. Fermanagh. 618 
In the face of the adjacent escarpment there is a very good 
section of a vertical sheet of chert exposed ; it measures seven 
feet vertically, and is four inches in breadth at top, thickening 
to eleven inches at the bottom, where it passes into the mass of 
rock beneath. Below this section, in the face of the cliff, 
horizontal layers of chert, as wellas nodules of the same, may be 
seen. 
During the process of disintegration the chert sheets, whether 
vertical or horizontal, weather in a similar fashion, some of the 
fragments which have crumbled off being rudely and irregularly 
prismatic, whilst others have come off in thin laminated plates. 
The character of the chert seems similar in all of the forms ; 
this point, however, cannot well be determined without 
examination of sections under the microscope. 
A question naturally arises here : were the vertical chert sheets 
formed simultaneously with the lmestone in which they are 
found, or were they afterwards formed in fissures produced in 
the limestone when it became hardened, by siliceous matter 
which might have been washed into cracks in the mass of 
limestone? The evidence connected with them points to the 
former conclusion, viz., that they were formed simultaneously 
with the limestone. 
There are adjacent rocks (part of the same formation) where 
chert is absent which attain a higher elevation, and which 
evidently extended at one time over the chert sheets in question, 
but whose extension has been removed by denudation. These 
elevated rocks form part of the adjoining cliff, and the beds com- 
posing them are in one unbroken sequence from bottom to top, 
and contain the fossils usually found in mountain limestone. 
The rocks containing the vertical sheets of chert, are exactly 
similar, and free from faults, cracks, or fissures. The horizontal 
and vertical sheets of chert appear to be one formation deposited 
in the bottom of the ancient Carboniferous sea. Still the question 
is not answered—how is it that some of the chert sheets are 
vertical ? 
