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