Nov. 9, 1882 | 
NATURE | 
45 
accumulation at the present day enables us at once to recognise, 
in the conglomerate sandstone and shale of our section, simply 
the consolidated sediments of earlier times. The occurrence of 
fossils in the strata determines whether the deposits were formed 
in fresh water, brackish water, or the sea—whether near to a 
coast-land, or at a greater distance from the shore, and so forth. 
If some of the fossils be of terrestrial origin, while others are 
brackish- water and marine, we gain not only certain knowledge 
of the life of the period, but, if the evidence be full enough, we 
may even form more or less reliable conclusions as to the physical 
and climatic conditions which at one time existed in the locality 
under our examination, In short there are many almost obvious 
conclusions, as we may term them, which the appearances pre- 
sented by an individual exposure of rocks must suggest to any 
observer who has previously become familiar with the operation 
of the natural forces in the world around us, He simply com- 
pares the facts with what is now taking place, and is thus led to 
conclude that effects the same in kind have been produced in the 
same way. 
Sometimes, however, the rocks present appearances which are 
harder to interpret in this obvious and ready manner. We en- 
counter, for example, a rock-mass having none of the features 
presented by ordinary sedimentary strata. Instead of being 
made up, like conglomerate and sandstone, of rounded stones or 
grains, arranged in layers, it is entirely composed of Jarger and 
smaller crystalline particles, not lying in lines and layers, but 
scattered indiscriminately through the whole mass. It does not 
occur in beds like ordinary sedimentary strata, but on the con- 
trary we see it cutting, as it were, across other rocks, and sending 
ont veins which penetrate the latter in all directions. The ob- 
server immediately concludes that the crystalline rock is of 
younger age than the beds traversed by it ; and not only so, but 
that the whole mass with all its veins was injected into its 
present position in a liquid, semi-liquid, or pasty, and probably 
heated condition. And in confirmation of this last conclusion 
he may perhaps note that the rocks immediately adjoining the 
dykes and veins betray the appearance of having been subjected 
to the action of heat. The grits, we shall suppose, are hardened 
and much cracked and shattered, and the shales baked and por- 
cellanised ; both rocks, when closely examined, showing traces 
of an incipient fusion along the line of contact with the intrusive 
rock, They may even lose their original granular texture, and 
assume a more or less crystalline aspect for some distance away 
from the dykes and veins that intersect them. All these features 
the observer may have seen exemplified in a modern volcanic 
district, and he may therefore feel justified in the opinion he has 
formed as to the formerly molten state and therefore igneous 
origin of our crystalline rock. His induction, however, is not 
complete. He compares his supposed igneous rock with the 
undoubted products of existing volcanoes, and although many 
of these last send out dykes and veins, and have a crystalline 
texture, yet not a single one may have any further resemblance 
to the crystalline rock of his section. He cannot, therefore, be 
any longer certain that his dykes and veins have originated in 
the same way as those of Etna and Vesuvius. ‘The origin of 
such a crystalline rock as I am speaking of (which we may sup- 
pose is granite), cannot be determined, like that of conglomerate 
or sandstone, by direct comparison with similar rocks in process 
of formation. Exhaustive examination of the granite itself, an 
intimate knowledge of its ingredients, and the conditions of 
formation which these imply, combined with careful observation 
of the mode in which this rock occurs wherever it is met with— 
these and other studies must be prosecuted before any assurance 
can be obtained as to the precise mode in which granitic dykes 
and veins have originated. The observer then learns that these 
are really of igneous origin, as he at first inferred, but his notion 
that they have been injected into strata at or near the surface 
like the dykes of modern volcanoes, cannot, he finds, be main- 
tained. All the evidence supplied by careful microscopical ex- 
amination and physical considerations, leads to the conclusion 
that granite has been formed and consolidated at considerable 
depths. Having satisfied himself upon this point, the observer 
will readily conclude that the dykes and veins that now appear 
at the surface were formerly buried under great masses of rock, 
which have since been removed. Of course there are many 
other facts connected with the history of granite which I do not 
touch upon at present. By and by we shall learn that all masses 
of granite are not intrusive, but that certain considerable areas 
of this rock, although agreeing in composition with the granite 
of dykes and veins, are nevertheless not irruptive. 
The conclusion that granite is of deep-seated origin is not, you 
will observe, contrary to our canon that the past is to be inter- 
preted by the present. Molten rock, as we know, is forced into 
fissures in the neighbourhood of active volcanoes, and there 
consolidated, and chemical analyses show that some volcanic 
rocks have the same ultimate composition as granite. Partly by 
observation and partly by experiment we detect in granite eyi- 
dence to prove that it has consolidated under pressure, and that, 
had the original molten mass cooled more rapidly and under 
moderate pres-ure, the resulting rock would have presented a 
very different appearance. Had injection taken place at or near 
the surface, or had the melted matter flowed out of a volcanic 
orifice, it might well have resembled some of the products of 
modern volcanoes, 
Let us now take another sample of the mode of interpreting 
geological phenomena. We shall go back to our section of 
conglomerate, sandstone, and shale—and these deposits we shall 
suppose belong to a comparatively recent date—to the Tertiary 
period, let us say. Suppose, moreover, that the fossils are 
numerous, and so well preserved that we are enabled to compare 
them with living forms. A few, we find, belong to existing 
species, others are closely related to these, while yet others, 
although without doubt extinct, can nevertheless be referred with 
confidence to living genera, ‘These facts enable us to come to 
a trustworthy conclusion as to former climatic conditions, for 
all we have to do is to examine the conditions under which the 
existing species presently flourish, and draw the obvious inference. 
Of course the larger the number of living species, and the more 
highly organised these are, the more reliable our theoretical 
results will be. But suppose our fossils indicate a warm and 
genial climate, and that the locality in which we discover our 
section lies far within the Arctic Cirele—what must our conclu- 
sion be? Simply this: that the climate of those high Jatitudes 
was formerly much warmer than it is now. We appeal to the 
present, and that is the reply we get. But the next question 
arises: How could such a climate obtain within the Arctic 
Circle? Thisis one of those crucial cases which must eventually 
determine whether Uniformitarians are justified in maintaining 
that the present is the exact type of all that has gone before, 
within known geological periods. According to them it is not 
necessary to look beyond this earth itself for an explanation ot 
such an apparent anomaly as the occurrence of southern faunas 
and floras in the Arctic regions. All we have to assume, they 
tell us, is a former very different distribution of land and water. 
They refer us to the well-proved fact that there have been fre- 
qnent considerable elevations and depressions of the land, which 
must have indirectly affected the climate of wide areas by modi- 
fying the course of oceanic and aérial currents, They argue 
that were the larger land-areas of the globe to be grouped about 
the equator, with oceanic islands scattered over the higher 
latitudes, this arrangement of land would induce all the condi- 
tions that are neccssary to account for the former growth of 
walnuts and oaks and beeches within the Arctic Circle. 
This hypothesis is opposed by others who maintain that no 
such distribution of land and water existed at the epoch in 
question. According to them, the position of the main con- 
tinental ridges and oceanic depressions was established at a very 
early period in the earth’s history. The persistence of these 
main features, however, does not imply a total invariability of 
outline. On the contrary, the protuberant areas, it is admitted, 
have been modified again and again all through the geological 
ages—considerable portions having been alternately depressed 
below and lifted above the sea-level. But as the relative posi- 
tions of the more important ridges and depressions of the earth’s 
surface—the continental areas and oceanic basins—were deter- 
mined long anterior to the deposition of the Tertiary strata, and 
probably date back to azoic times—such a re-arrangement of 
land and sea as the Uniformitarian view requires cannot have 
taken place. It is further maintained that, even could such a 
re-arragement be substituted for the present, it would not bring 
about a genial climate in the Arctic regions. We must look 
beyond our globe itself, we are told, if we wish to find the key 
to those greater revolutions of climate of which we have evi- 
dence in such a case as the occurrence of a southern flora within 
the Arctic Circle. The greater climatic revolutions of the past 
are due, we are assured, to periodical changes in the eccentricity 
of the earth’s orbit, combined with the precession of the equi- 
noxes, and the influence which such mutations must have exerted 
upon the ordinary agents of geological change. 
The soundness of these opposing views must of course be 
