> 
40 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
described as the dack-bone of England. It attains its highest 
elevation of about 2,600 feet in the North Riding of Yorkshire ; 
and in the Peak, the highest part of North Derbyshire, it reaches 
about 2,000 feet above the sea-level. When followed from the 
northern counties southward this ridge of high land is found to 
terminate abruptly, a few miles to the west of Ashbourne, in the 
Weaver Hills ; and it is this southern portion of the range which 
serves to illustrate, in the best possible way, the geological 
structure of the Pennine Chain. 
The rocks, constituting the Chain, belong exclusively to the 
Carboniferous System—that great division of the older rocks 
which, in its upper part, contains all the principal coal seams 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 A//stone 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. 
Upper 
Coat Measures | Middle 
UpprpER CARBONIFEROUS Lower 
MILLSTONE GRIT 
YOREDALE SHALES 
Mountain LIMESTONE. 
The exact line drawn between these various members of the 
LOWER Cantossensous| 
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. 
