RESEARCHES AMONG THE PALAEOZOIC ROCKS OF IRELAND. 123 
stone, or any other rocks, and whether gray, green, red, or brown. 
I look upon this band of conglomerate as a most important land¬ 
mark, or index, in Geology; it is the boundary between two distinct 
periods of organic life, and, besides the changes in the genera and 
species of the animals that existed below and above it, there is a 
well-marked difference in the lithological character of the rocks 
also—the more ancient being much harder. Those hard grits, crys¬ 
talline limestones, and clay slates, of the older period, have their re¬ 
presentatives in the soft sandstones, ordinary limestones, and shales 
of the newer. 
Grit and sandstone are two names which appear to be often used, 
one for the other, in Geology. Grit, in this paper, means a very 
hard, quartzose, rock, often green or gray, sometimes dark brown, 
and found in the older and lower sedimentary rocks, interstratified 
with green, gray, or purple slates, having a distinct cleavage. 
Those grits are generally very refractory under the hammer or 
chisel of the workman. Sandstone, on the contrary, is a soft, sandy 
rock, easily split into rectangular blocks, or chiselled for economic 
purposes. Sometimes it is red, sometimes yellow or white. Such 
in Ireland is usually got in the Old Red Sandstone, and in the Coal 
series. 
In the foregoing Table, also, some of the localities exhibit points 
of geological interest, to those who may visit them, worthy of a few 
remarks. I shall notice them separately and consecutively, in the 
order in which they are numbered at the end of this paper. 
In treating of this subject, it appears desirable to make an at¬ 
tempt at determining the thickness of this division of the Carboni¬ 
ferous system. It is not easy to get good sections of it, for it fre¬ 
quently happens that the bottom or the top is not clearly visible; the 
rock not always well exposed from being covered with drift, or there 
may be a fault, and consequent downthrow of the strata, and the 
surface made even afterwards by denudation, making it impossible 
to detect the fault. Therefore, the thicknesses resulting from sec¬ 
tions can seldom be depended on for anything more than an ap¬ 
proximation to the truth. I have selected a few of the best sections 
I know, and, from the heights given on the Ordnance maps, laid 
them down on the same scale of length and height, in the shortest 
line across the strike; then, measuring the thickness from the bottom 
