In each country that he examines, he finds
that certain groups of strata lie above certain other groups;
and in comparing different countries with one another, he finds
that, in the main, the same groups of rocks are always found in the
same relative position to each other. It is possible, therefore,
for the physical geologist to arrange the known stratified rocks
into a successive series of groups, or "formations," having a
certain definite order. The establishment of this physical order
amongst the rocks introduces, however, at once the element of
_time_, and the physical succession of the strata can be converted
directly into a historical or _chronological_ succession. This
is obvious, when we reflect that any bed or set of beds of
sedimentary origin is clearly and necessarily younger than all
the strata upon which it rests, and older than all those by which
it is surmounted.
It is possible, then, by an appeal to the rocks alone, to determine
in each country the general physical succession of the strata,
and this "stratigraphical" arrangement, when once determined,
gives us the _relative_ ages of the successive groups. The task,
however, of the physical geologist in this matter is immensely
lightened when he calls in palaeontology to his aid, and studies
the evidence of the fossils embedded in the rocks. Not only is
it thus much easier to determine the order of succession of the
strata in any given region, but it becomes now for the first time
possible to compare, with certainty and precision, the order of
succession in one region with that which exists in other regions
far distant.
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