182 GENERAL VIEWS. 
in the coal tracts above millstone grit. It is usually sulphureous, and 
contains oxide of iron, but some thin layers are crystalline and pure 
as any other coal. Specific gravity (see page 130) according to the 
purity of the coal 1*29 to L62. 
A ature of the series. Reduced to its greatest simplicity the carbo- 
niferous system presents three leading groups, viz. coal measures, aver- 
age sp. gr. 2-4 ; mountain limestone 27 ; old red sandstone 2'5. This 
is nearly exemplified in the South of England. The next modification, 
fouud in Wales, Derbyshire, and Yorkshire, admits of two intermediate 
transition groups, which really connect the other three into one com- 
pound system. See the Tables, p. p. 11 , and 12. 
As we proceed northward these groups become further subdivided, 
but, the number of the component substances remaining the same, a 
general analogy still remains, and the most complicated details are 
reducible to the same general scheme : thus. 
Coal measures Coal measures 
Millstone grit 
Mountain limestone Mountain limestone 
Limestone and red sandstone 
Old red sandstone Red sandstone. 
The Yoredale series of rocks where fully developed exhibits five 
principal and similar terms ; and several irregular terms, which may be 
thus arranged, 
Sp. Gr. 
{ Limestone ... 2-6 — 2*7 
Grit or flagstone 2-5 
Plate ... 2-5 
' Upper millstone grit. 
Various. 
Middle millstone grit. 
Various. 
Lower millstone grit. 
Various. 
( Yoredale rocks. 
( Wharfedale series. 
Main limestone, &c. 
