
Bg: = SY oo 
632 STRATIGRAPHICAL GEOLOGY.  [Boox VI. — 
formations, till they too cease, and their places are taken by yet — 
older forms. Itis thus shown that the stratified rocks contain the — 
records of a gradual progression of organic types. A species which. 
has once died out does not seem ever to have reappeared. But as 
has been already pointed out in reference to Barrande’s doctrine of 
colonies, a species may within a limited area appear in a formation 
older than that of which it is elsewhere characteristic, having 
temporarily migrated into the district from some neighbouring 
region where it had already established itself. 
4, When the order of succession of organic remains among the 
stratified rocks has once been accurately determined, it becomes an 
invaluable guide in the investigation of the relative age and struc- 
tural arrangements of rocks. ach zone or group of strata, being 
characterized by its own species or genera, may be recognized by 
their means, and the true succession of strata may thus be confidently 
established even in a country wherein the rocks have been greatly 
fractured, folded, or inverted. 
5. The relative chronological value of the divisions of the 
Geological Record is not to be measured by mere depth of strata. 
While a great thickness of stratified rock may be reasonably assumed 
to mark the passage of a long period of time, it cannot safely be 
affirmed that a much less thickness elsewhere represents a corre- 
spondingly diminished period. The truth of this statement may 
sometimes be made evident by an unconformability between two sets 
of rocks, as has already been explained. The total depth of both 
eroups together may be, say 1000 feet. Hlsewhere we may find a 
single unbroken formation reaching a depth of 10,000 feet; but it 
would be utterly erroneous to conclude that the latter represents ten 
times the duration indicated by the two former. So far from this 
being the case, it might not be difficult to show that the minor 
thickness of rock really denoted by far the longer geological interval. 
If, for instance, it could be proved that the upper part of both the 
sections lay on one and the same geological platform, but that the 
lower unconformable series in the one locality belonged to a far 
lower and older system of rocks than the base of the thick conform- 
able series in the other, then it would be clear that the gap marked 
by the unconformability really indicated a longer period than the 
massive succession of deposits. 
6. Iossil evidence furnishes the chief means of comparing the 
relative chronological value of groups of rock. A break in the 
succession of organic remains marks an interyal of time often un-— 
represented by strata at the place where the break is found. The 
relative importance of these breaks, and therefore, probably, the 
comparative intervals of time which they mark, may be estimated 
by the difference of the facies of the fossils on each side. If, for 
example, in one case we find every species to be dissimilar above and 
below a certain horizon, while in another locality only half of the 

