Geological Observations on South America | Page 7

will be found scattered through the chapter.
Darwin's study of the older tertiary formations, with their abundant
shells, and their relics of vegetable life buried under great sheets of
basalt, led him to consider carefully the question of climate during
these earlier periods. In opposition to prevalent views on this subject,
Darwin points out that his observations are opposed to the conclusion
that a higher temperature prevailed universally over the globe during
early geological periods. He argues that "the causes which gave to the
older tertiary productions of the quite temperate zones of Europe a
tropical character, WERE OF A LOCAL CHARACTER AND DID
NOT AFFECT THE WHOLE GLOBE." In this, as in many similar
instances, we see the beneficial influence of extensive travel in freeing
Darwin's mind from prevailing prejudices. It was this widening of
experience which rendered him so especially qualified to deal with the
great problem of the origin of species, and in doing so to emancipate
himself from ideas which were received with unquestioning faith by
geologists whose studies had been circumscribed within the limits of
Western Europe.
In the Cordilleras of Northern and Central Chili, Darwin, when
studying still older formations, clearly recognised that they contain an
admixture of the forms of life, which in Europe are distinctive of the
Cretaceous and Jurassic periods respectively. He was thus led to
conclude that the classification of geological periods, which fairly well

expresses the facts that had been discovered in the areas where the
science was first studied, is no longer capable of being applied when
we come to the study of widely distant regions. This important
conclusion led up to the further generalisation that each great
geological period has exhibited a geographical distribution of the forms
of animal and vegetable life, comparable to that which prevails in the
existing fauna and flora. To those who are familiar with the extent to
which the doctrine of universal formations has affected geological
thought and speculation, both long before and since the time that
Darwin wrote, the importance of this new standpoint to which he was
able to attain will be sufficiently apparent. Like the idea of the extreme
imperfection of the Geological Record, the doctrine of LOCAL
geological formations is found permeating and moulding all the
palaeontological reasonings of his great work.
In one of Darwin's letters, written while he was in South America, there
is a passage we have already quoted, in which he expresses his inability
to decide between the rival claims upon his attention of "the old
crystalline group of rocks," and "the softer fossiliferous beds"
respectively. The sixth chapter of the work before us, entitled "Plutonic
and Metamorphic Rocks--Cleavage and Foliation," contains a brief
summary of a series of observations and reasonings upon these
crystalline rocks, which are, we believe, calculated to effect a
revolution in geological science, and-- though their value and
importance have long been overlooked--are likely to entitle Darwin in
the future to a position among geologists, scarcely, if at all, inferior to
that which he already occupies among biologists.
Darwin's studies of the great rock-masses of the Andes convinced him
of the close relations between the granitic or Plutonic rocks, and those
which were undoubtedly poured forth as lavas. Upon his return, he set
to work, with the aid of Professor Miller, to make a careful study of the
minerals composing the granites and those which occur in the lavas,
and he was able to show that in all essential respects they are identical.
He was further able to prove that there is a complete gradation between
the highly crystalline or granitic rock-masses, and those containing
more or less glassy matter between their crystals, which constitute

ordinary lavas. The importance of this conclusion will be realised when
we remember that it was then the common creed of geologists--and still
continues to be so on the Continent--that all highly crystalline rocks are
of great geological antiquity, and that the igneous ejections which have
taken place since the beginning of the tertiary periods differ essentially,
in their composition, their structure, and their mode of occurrence, from
those which have made their appearance at earlier periods of the
world's history.
Very completely have the conclusions of Darwin upon these subjects
been justified by recent researches. In England, the United States, and
Italy, examples of the gradual passage of rocks of truly granitic
structure into ordinary lavas have been described, and the reality of the
transition has been demonstrated by the most careful studies with the
microscope. Recent researches carried on in South America by
Professor Stelzner, have also shown the existence of a class of highly
crystalline rocks--the "Andengranites"--which combine in themselves
many of the characteristics which were once thought to be distinctive of
the so-called Plutonic and volcanic rocks. No one familiar with recent