12 
DESCRIPTION OF THE ROCKS 
pletely united by intermediate and almost indefinite alternation of 
sandstone, limestone, shale, coal, and ironstone. Though in some 
countries the old red conglomerate shews no sign of union to the 
mountain limestone, yet near Bristol,- round all the region of the 
Cumbrian slate, along the Penine chain, and in Tweeddale, the al- 
ternation of red sandstone and mountain limestone is a common and 
characteristic phenomenon. Such a series of strata is in fact double, 
and belongs equally to the superior and inferior groups ; and instead 
of being considered neutral, or named as a distinct series, should 
rather be included in both, as indicated by the mode of arranging 
the brackets in the table just given. 
Groups or series, quite distinct in their middle terms as limestone 
and shale, may yet graduate into one another, so as to leave no line 
of demarcation , but a band of transition deposits. This phenomenon 
depends on the same general principle as the union of deposits by 
alternation, — viz. a change of the physical conditions of the region ; 
the difference between the two orders of effects produced in one 
period, may hereafter reveal to us the geographical circumstances of 
the deposits, especially their distance from the centre of the physical 
disturbance. 
Transition groups formed by gradation or alternation of the terms 
of any series may be thus represented. Let c., 1., r., stand for coal, 
limestone, and red sandstone. 
Gradation of substance. 
Coal formation = C + C -f- C 
m • . f C -f- 1 -|- C 
Transition group — j i _j_ c j 
Mountain limestone for-__ 1 l _l L, i l 
mation j 
, . f 1 -f- r ~j~ 1 
Transition group = j r q_ i r 
Red sandstone formation = R + R + R 
Alternation of hah. 
Coal formation = C + C+ C 
Transition group 
Mountain limestone for- 
mation 
= n + i + i + i 
i c +c 
} 
c + C+ c 
L + L + L 
_ fr + r + r + r 
~ 11 + 1 + 1 +1 
Transition group 
Red sandstone formations R -(- R -}- R 
