62 
FORMATIONS. 
if occupied by clay and soft materials, a clay dike. 
How these fissures came to be filled by such sub- 
stances will be a subject of inquiry hereafter. They 
vary in thickness from a few inches to hundreds of 
feet. 
When a series of strata of a similar rock are ar- 
ranged with occasional intervening strata of rocks 
of another kind, which recur in different parts of 
the series, they are regarded as having been all 
formed nearly at the same epoch and under similar 
circumstances ; and such beds are called by geolo- 
gists formations. Thus the strata of shale, sand- 
stone, and ironstone that accompany beds of coal, 
are called the coal formation. Strata of, different 
kinds, in which a gradation is observed into each 
other, and which contain similar species of organic 
remains, also constitute a geological formation. The 
chalk with flints, the lower chalk without flints, the 
chalk marl, and the green sand under the chalk, are 
regarded as members of what is denominated the 
chalk formation.^ There is one circumstance of 
frequent occurrence which is apt to confound the 
young geologist, and that is, in travelling in mount- 
ainous districts, after passing over a certain series 
of rocks, he again meets with them in a contrary 
order. How this is accounted for figure 15 will help 
to explain. 
Fig. 15. 
C c are parts of the lowest stratified rocks, aa oi 
the highest Now suppose them to be lying in a 
* Bakewell*s Geology. 
