Parr X.] UNCONFORMABILITY. ' 601 
_ Fig. 312, the lower series of beds (¢) has been upturned and denuded 
_ before the deposition of the upper series (a b) upon them. In this 
instance the upper worn surface of the limestones c has been perforated 
by boring molluscs below the sandy stratum (0). 

Fic. 312.— UNcONFORMABILITY BETWEEN HORIZONTAL AND INCLINED STRATA. INFERIOR 
OoLite (a b) RESTING ON CARBONIFEROUS LIMESTONE (¢c), FROME, SoMERSET (B.). 
An unconformability forms one of the great breaks in the geo- 
logical record. In Fig. 213 (p. 495), by way of illustration, we see 
at once that a notable hiatus in deposition, and therefore in 
geological chronology, must exist between the older conformable 
series, a 6 c, and the later strata by which these are covered. The 
former had been deposited, folded, upheaved, and worn down before 
the accumulation of the newer series upon their denuded edges. 
These changes must have demanded a considerable lapse of time. 
Yet, looking merely at the structure in itself, we have evidently 
no means of fixing, even relatively, the length of interval marked 
by an unconformability. By ascertaining from some other region 
the full suite of formations we learn what members of the succession 
are wanting. In this way it would be discovered that the greater 
part of the Carboniferous system, the whole of the Permian, and the 
Trias up to the base of the Lias are absent from the ground repre- 
sented in Fig. 311. The mere violence of contrast between a set of 
vertical beds below and a horizontal group above is in itself no 
certainly reliable criterion of the relative lapse of time between their 
deposition, for obviously an older portion of a given formation might 
be tilted on end and be overlaid unconformably by a later part of 
the same formation. A set of flat rocks of high geological antiquity 
may, on the other hand, be conformably covered by a formation of 
comparatively recent date, yet in spite of the want of discordance 
between the two, they might have been separated by a large portion 
of the total sum of geological time. 
Further examination will usually suffice ==42-=> ==] See 
to show that the eau ue Wee WWE, Ws 
cases is only partial or accidental, an Le 
that ices may be found where the Be eS eee 
formations are distinctly unconform- 
able. From the centre of the section in Fig. 318, for example, the 
two groups of rocks might on casual examination be pronounced to 
be conformable. Yet at short distances on either side proofs of 
violent unconformability are conspicuous. It sometimes happens 

