ic origin,
though this is not capable of direct proof. When present, at any
rate, in quantity, and in the form of layers associated with
stratified rocks, as is often the case in the Laurentian formation,
there can be little hesitation in regarding it as of vegetable
origin, and as an altered coal.

[Footnote 7: In the Huronian formation at Steel River, on the
north shore of Lake Superior, there exists a bed of carbonaceous
matter which is regularly interstratified with the surrounding
rocks, and has a thickness of from 30 to 40 feet. This bed is
shown by chemical analysis to contain about 50 per cent of carbon,
partly in the form of graphite, partly in the form of anthracite;
and there can be little doubt but that it is really a stratum
of "metamorphic" coal.]

CHAPTER III.

CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.

The physical geologist, who deals with rocks simply as rocks,
and who does not necessarily trouble himself about what fossils
they may contain, finds that the stratified deposits which form
so large a portion of the visible part of the earth's crust are
not promiscuously heaped together, but that they have a certain
definite arrangement. 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,