THE PHYSICAL GEOGRAPHY OF THE PAST. 
201 
Upper Carboniferous 
Lower Carboniferous 
( 
/ 
Coal Measures 
Middle. 
Lower. 
of this country. This division, or system , as geologists call 
it, can be broadly divided into four series of strata which are 
invariably found in the same vertical order or succession. 
The lowest series is that of the Mountain Limestone, a mass of 
hard, and in some cases almost crystalline, Limestone rock. 
Above the Mountain Limestone we have a series of black 
shales and sandstones, with thin beds of impure limestone, 
known as the Yoredale Shales: and above the Yoredale Shales 
we have the Millstone Grit, a mass of coarse sandstones with 
thick shale partings, and an occasional thin seam of impure 
coal. The Millstone Grit is in turn overlaid by the Upper, 
Middle, and Lower Coal Measures, which are made up of an 
assemblage of sandstones and shales, with many important 
and excellent beds of workable coal in their middle division. 
j Upper. 
1 
Millstone Grit. 
Yoredale Shales. 
] Mountain Limestone. 
The exact line drawn between these various members of 
the Carboniferous System is, for the most part, an arbitrary 
one, for although, when looked at as a whole, each division 
has very different characters from the one below and above it, 
yet they often pass into each other vertically by insensible 
gradations. 
The total thickness of this great pile of strata is probably 
over 12,000 feet in Derbyshire, and this does not represent 
the whole of the original thickness of the beds. From the 
evidently sedimentary character of all these rocks we are 
quite sure that the materials were laid down under water in 
a horizontal, or almost horizontal, position, but we now find 
them elevated considerably above the level of the sea, and 
with their original bedding lines inclined to the horizon at all 
sorts of angles. The true relation of such beds to each 
other, and to the present conformation of the surface, can 
only be ascertained when a large district has been surveyed, 
and all the outcrops of the strata laid down on a map with 
the observed inclinations or dip of the strata, as it is called. 
From such maps it is possible to construct imaginary sections 
across a country, showing at a glance the present position 
of strata, which, once forming horizontal and continuous 
sheets, may now have become folded, faulted, and discon¬ 
nected in all sorts of complicated ways. 
You have before you such a horizontal section, taken 
along an east and west line across the Pennine flange, from 
