738 STRATIGRAPHICAL GEOLOGY. — [Book VI. 
Northumberland southwards to the low plains in the centre of England 
there runs a ridge of high ground, formed by a great anticline, along 
which the Carboniferous Limestone appears at intervals from under- 
neath higher members of the system. In this northern Carboniferous area, 
of which the axis is known as the Pennine Chain, the limestone attains its 
greatest development. In one portion of the district it reaches a depth 
of 4000 feet, and yet its actual base is nowhere seen. This Pennine 
region appears to have been the area of maximum depression during the 
early part of the Carboniferous period in Britain. Traced towards the 
south-west, the limestone diminishes to sometimes not more than 500 
feet in South Wales. Northwards, losing its character as a massive 
calcareous formation, it is split up by intercalations of sandstone, shale, 
coal, &c., until actual limestone becomes a very subordinate member of 
the series in central Scotland. 
Where typically developed, the Carboniferous Limestone is a massive 
well-bedded limestone, chiefly light bluish-grey in colour, varying from — 
a compact homogeneous to a distinctly crystalline texture, and rising into 
ranges of hills, whence its original name “ Mountain Limestone.” It 
contains occasional scattered irregular nodules and nodular beds of dark 
chert (phtanite). Though it is abundantly fossiliferous, little has yet 
been done in working out in detail the successive life-zones of this great 
mass of rock, as has been done so well for the corresponding lime- 
stone series of Belgium. The fossils commonly stand out on weathered 
surfaces of the rock, but microscopic investigation shows that even 
those portions of the mass which appear most structureless consist 
of the crowded remains of marine organisms. The limestone may be 
regarded as derived almost entirely from the organic debris of a sea- 
floor. Diversities of colour and lithological character occur, whereby 
the bedding of the thick calcareous mass can be distinctly seen. Here 
and there a more marked crystalline structure has been superin- 
duced ; while along lines of principal joints the rock on either side for a 
breadth of 20 or 30 fathoms is converted into yellowish or brown dolomite 
or ‘‘dunstone” (see p. 305). In Derbyshire, sheets of contemporaneous 
lava, locally termed ‘“ toadstone,” are interpolated in the Carboniferous 
Limestone. Other evidences of contemporaneous volcanic action have 
been noted by Mr. J. Horne in the Isle of Man, but it is in Scotland, as 
will be immediately referred to, that the most remarkable proofs of 
abundantly active Carboniferous volcanoes have been preserved. 
In the Carboniferous areas of the south-west of England and South 
Wales, the limits of the Carboniferous Limestone are well defined by the 
Limestone Shale below, and by the Farewell Rock or Millstone Grit above. 
In the Pennine area, however, the massive limestone is succeeded by a 
series of shales, limestones, and sandstones, known as the Yoredale 
group. These cover a large area and attain a great thickness. In North 
Staffordshire they are 2300 feet thick, which, added to the 4000 feet of 
limestone below, gives a depth of 6300 feet for the whole Carboniferous 
Limestone series of that region. In Lancashire the Yoredale rocks 
attain still more stupendous dimensions, Mr. Hull having found them to 
be no less than 4500 feet thick. Both the lower or main (Scaur) lime- 
stone and the Yoredale group pass northwards into sandstones and 
shales, with coal-seams, and diminish in thickness. 
Traced northwards into Scotland the Carboniferous Limestone under- 
goes a remarkable petrographical and paleontological change. Its 
