Some years before the voyage of the "Beagle," Mr. Poulett Scrope had
pointed out the remarkable analogies that exist between certain igneous
rocks of banded structure, as seen in the Ponza Islands, and the foliated
crystalline schists. It does not appear that Darwin was acquainted with
this remarkable memoir, but quite independently he called attention to the
same phenomena when he came to study some very similar rocks which occur in
the island of Ascension. Coming fresh from the study of the great masses of
crystalline schist in the South American continent, he was struck by the
circumstance that in the undoubtedly igneous rocks of Ascension we find a
similar separation of the constituent minerals along parallel "folia."
These observations led Darwin to the same conclusion as that arrived at
some time before by Scrope--namely that when crystallisation takes place in
rock masses under the influence of great deforming stresses, a separation
and parallel arrangement of the constituent minerals will result. This is a
process which is now fully recognised as having been a potent factor in the
production of the metamorphic rock, and has been called by more recent
writers "dynamo-metamorphism."

In this, and in many similar discussions, in which exact mineralogical
knowledge was required, it is remarkable how successful Darwin was in
making out the true facts with regard to the rocks he studied by the simple
aid of a penknife and pocket-lens, supplemented by a few chemical tests and
the constant use of the blowpipe. Since his day, the method of study of
rocks by thin sections under the microscope has been devised, and has
become a most efficient aid in all petrographical inquiries. During the
voyage of H.M.S. "Challenger," many of the islands studied by Darwin have
been revisited and their rocks collected. The results of their study by one
of the greatest masters of the science of micropetrography--Professor
Renard of Brussels--have been recently published in one of the volumes of
"Reports on the 'Challenger' Expedition." While much that is new and