Geological Observations on South America | Page 8

geological literature--even in Germany and France, where the old views
concerning the distinction of igneous products of different ages have
been most stoutly maintained--can fail to recognise the fact that the
principles contended for by Darwin bid fair at no distant period to win
universal acceptance among geologists all over the globe.
Still more important are the conclusions at which Darwin arrived with
respect to the origin of the schists and gneisses which cover so large an
area in South America.
Carefully noting, by the aid of his compass and clinometer, at every
point which he visited, the direction and amount of inclination of the
parallel divisions in these rocks, he was led to a very important
generalisation-- namely, that over very wide areas the direction (strike)
of the planes of cleavage in slates, and of foliation in schists and
gneisses, remained constant, though the amount of their inclination (dip)
often varied within wide limits. Further than this it appeared that there
was always a close correspondence between the strike of the cleavage

and foliation and the direction of the great axes along which elevation
had taken place in the district.
In Tierra del Fuego, Darwin found striking evidence that the cleavage
intersecting great masses of slate-rocks was quite independent of their
original stratification, and could often, indeed, be seen cutting across it
at right angles. He was also able to verify Sedgwick's observation that,
in some slates, glossy surfaces on the planes of cleavage arise from the
development of new minerals, chlorite, epidote or mica, and that in this
way a complete graduation from slates to true schists may be traced.
Darwin further showed that in highly schistose rocks, the folia bend
around and encircle any foreign bodies in the mass, and that in some
cases they exhibit the most tortuous forms and complicated puckerings.
He clearly saw that in all cases the forces by which these striking
phenomena must have been produced were persistent over wide areas,
and were connected with the great movements by which the rocks had
been upheaved and folded.
That the distinct folia of quartz, feldspar, mica, and other minerals
composing the metamorphic schists could not have been separately
deposited as sediment was strongly insisted upon by Darwin; and in
doing so he opposed the view generally prevalent among geologists at
that time. He was thus driven to the conclusion that foliation, like
cleavage, is not an original, but a superinduced structure in rock-masses,
and that it is the result of re-crystallisation, under the controlling
influence of great pressure, of the materials of which the rock was
composed.
In studying the lavas of Ascension, as we have already seen, Darwin
was led to recognise the circumstance that, when igneous rocks are
subjected to great differential movements during the period of their
consolidation, they acquire a foliated structure, closely analogous to
that of the crystalline schists. Like his predecessor in this field of
inquiry, Mr. Poulett Scrope, Charles Darwin seems to have been
greatly impressed by these facts, and he argued from them that the
rocks exhibiting the foliated structure must have been in a state of
plasticity, like that of a cooling mass of lava. At that time the

suggestive experiments of Tresca, Daubree, and others, showing that
solid masses under the influence of enormous pressure become actually
plastic, had not been published. Had Darwin been aware of these facts
he would have seen that it was not necessary to assume a state of
imperfect solidity in rock-masses in order to account for their having
yielded to pressure and tension, and, in doing so, acquiring the new
characters which distinguish the crystalline schists.
The views put forward by Darwin on the origin of the crystalline
schists found an able advocate in Mr. Daniel Sharpe, who in 1852 and
1854 published two papers, dealing with the geology of the Scottish
Highlands and of the Alps respectively, in which he showed that the
principles arrived at by Darwin when studying the South American
rocks afford a complete explanation of the structure of the two districts
in question.
But, on the other hand, the conclusions of Darwin and Sharpe were met
with the strongest opposition by Sir Roderick Murchison and Dr. A.
Geikie, who in 1861 read a paper before the Geological Society "On
the Coincidence between Stratification and Foliation in the Crystalline
Rocks of the Scottish Highlands," in which they insisted that their
observations in Scotland tended to entirely disprove the conclusions of
Darwin that foliation in rocks is a secondary structure, and entirely
independent of the original stratification of the rock-masses.
Now it is a most significant circumstance that, no sooner did the
officers of the Geological Survey commence the careful and detailed
study of the Scottish Highlands than they found themselves compelled
to make a formal