240 THE COAL MEASURES. 
gradation from the one set of deposites to the 
other, and points to the curious fact that the pro- 
cesses which brought together the materials of the 
coal commenced before the previous movements 
that caused the conglomerate had wholly ceased. 
This very coarse aggregate has rarely more than 
one or two coal-beds below it ; and, ascending a little 
in the series, we find that its place is supplied by 
thick beds of shale and masses of soft argillaceous 
sandstones, some of whose layers have a sprink- 
ling of pebbles, which give the aspect of conglom- 
erates. These pebbles are smaller and more irreg- 
ular than those composing the rock at the very 
base of the series. The coal, and the slates imme- 
diately in contact with the coal, lie interstratified 
with these numerous coarse beds in an alternating 
group of great thickness. 
" Between the conglomerates, or even the coarser 
sandstones and the beds of coal, argillaceous sand- 
stones and blue shales are almost invariably inter- 
posed. The predominant rock of the upper part of 
the series is a compact blue sandstone, containing 
much argillaceous matter and oxide of iron, which 
cause the atmospheric agents to decompose it su- 
perficially, and to impart a dingy brown colour and 
a tendency to a conchoidal fracture, and to a sca- 
ling off at the corners. 
" The shales, which are next in importance to the 
argillaceous sandstones, are commonly of a dark 
blue or bluish gray colour when freshly broken; 
but many of them, by exposure to the atmosphere 
and to the vicissitudes of the seasons, assume a 
brownish ochreous hue, and crumble rapidly to 
pieces. Occasionally these shales contain highly 
ferruginous bands, in some of which occur layers of 
tolerably rich argillaceous iron ore. In the anthra- 
cite coal measures, as a general rule, this ore does 
not appear to exist in that abundance which it ex- 
hibits in many portions of the bituminous coal se- 
