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.