The Fact That Sir Roderick Murchison, As A Geologist, Laid Down A Theory
Of The Existence Of A Chain Of
Lakes upon an elevated plateau in Central
Africa, which theory has been now in great measure confirmed by actual
inspection,
Induces me to quote an extract from his address at the
anniversary meeting of the Royal Geographical Society, 23d May, 1864. In
that address, he expressed opinions upon the geological structure and
the races of Central Africa, which preceded those that I formed when at
the Albert lake. It is with intense interest that I have read the
following extract since my return to England: -
"In former addresses, I suggested that the interior mass and central
portions of Africa constituting a great plateau occupied by lakes and
marshes from which the waters escaped by cracks or depressions in the
subtending older rocks, had been in that condition during an enormously
long period. I have recently been enabled, through the apposite
discovery of Dr. Kirk, the companion of Livingstone, not only to fortify
my conjecture of 1852, but greatly to extend the inferences concerning
the long period of time during which the central parts of Africa have
remained in their present condition, save their degradation by ordinary
atmospheric agencies. My view, as given to this Society in 1852, was
mainly founded on the original and admirable geological researches of
Mr. Bain in the colony of the Cape of Good Hope. It was, that, inasmuch
as in the secondary or mesozoic age of geologists, the northern interior
of that country was occupied by great lakes and marshes, as proved by
the fossil reptile discovered by Bain, and named Dicynodon by Owen, such
it has remained for countless ages, even up to the present day. The
succeeding journeys into the interior, of Livingstone, Thornton and
Kirk, Burton and Speke, and Speke and Grant, have all tended to
strengthen me in the belief that Southern Africa has not undergone any
of those great submarine depressions which have so largely affected
Europe, Asia, and America, during the secondary, tertiary, and quasi
modern periods.
"The discovery of Dr. Kirk has confirmed my conclusion. On the banks of
an affluent of the Zambesi, that gentleman collected certain bones,
apparently carried down in watery drifts from inland positions, which
remains have been so fossilized as to have all the appearance of
antiquity which fossils of a tertiary or older age usually present. One
of these is a portion of the vertebral column and sacrum of a buffalo,
undistinguishable from that of the Cape buffalo; another is a fragment
of a crocodile, and another of a water-tortoise, both undistinguishable
from the forms of those animals now living. Together with these, Dr.
Kirk found numerous bones of antelopes and other animals, which, though
in a fossil condition, all belonged, as he assured me, to species now
living in South Africa.
"On the other hand, none of our explorers, including Mr. Bain, who has
diligently worked as a geologist, have detected in the interior any
limestones containing marine fossil remains, which would have proved
that South Africa had, like other regions, been depressed into oceanic
conditions, and re-elevated. On the contrary, in addition to old
granitic and other igneous rocks, all explorers find only either
innumerable undulations of sandstones, schistose, and quartzose rocks,
or such tufaceous and ferruginous deposits as would naturally occur in
countries long occupied by lakes and exuberant jungles, separated from
each other by sandy hills, scarcely any other calcareous rocks being
found except tufas formed by the deposition of landsprings. It is true
that there are marine tertiary formations on the coasts (around the Cape
Colony, near the mouth of the Zambesi opposite Mozambique, and again on
the coasts of Mombas opposite Zanzibar), and that these have been raised
up into low-coast ranges, followed by rocks of igneous origin. But in
penetrating into the true interior, the traveller takes a final leave of
all such formations; and in advancing to the heart of the continent, he
traverses a vast region which, to all appearance, has ever been under
terrestrial and lacustrine conditions only. Judging, indeed, from all
the evidences as yet collected, the interior of South Africa has
remained in that condition since the period of the secondary rocks of
geologists! Yet, whilst none of our countrymen found any evidences of
old marine remains, Captain Speke brought from one of the ridges which
lay between the coast and the lake Victoria N'yanza a fossil shell,
which, though larger in size, is undistinguishable from the Achatina
perdix now flourishing in South Africa. Again, whilst Bain found fossil
plants in his reptiliferous strata north of the Cape, and Livingstone
and Thornton discovered coal in sandstone, with fossil plants, like
those of our old coal of Europe and America, - yet both these mesozoic
and palaeozoic remains are terrestrial, and are not associated with
marine limestones, indicative of those oscillations of the land which
are so common in other countries.
"It is further to be observed, that the surface of this vast interior is
entirely exempt from the coarse superficial drift that encumbers so many
countries, as derived from lofty mountain-chains from which either
glaciers or great torrential streams have descended. In this respect, it
is also equally unlike those plains of Germany, Poland, and Northern
Russia, which were sea-bottoms when floating icebergs melted and dropped
the loads of stone which they were transporting from Scandinavia and
Lapland.
"In truth, therefore, the inner portion of Southern Africa is, in this
respect, as far as I know, geologically unique in the long conservation
of ancient terrestrial conditions. This inference is further supported
by the concomitant absence, throughout the larger portion of all this
vast area, i.e. south of the Equator, of any of those volcanic rocks
which are so often associated with oscillations of the terra firma
["Although Kilimandjaro is to a great extent igneous and volcanic, there
is nothing to prove it has been in activity during the historic era."]
"With the exception of the true volcanic hills of the Cameroons recently
described by Burton, on the west coast, a little to the north of the
Equator, and which possibly may advance southwards towards the Gaboon
country, nothing is known of the presence of any similar foci of
sub-aerial eruption all round the coasts of Africa south of the Equator.
If the elements for the production of them had existed, the coast-line
is precisely that on which we should expect to find such volcanic vents,
if we judge by the analogy of all volcanic regions where the habitual
igneous eruptions are not distant from the sea, or from great internal
masses of water.
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