This Certainly Is The True
Explanation, For I Found Numerous Shells Of Existing Species
On These Terraces.
Five narrow, gently sloping, fringe-like
terraces rise one behind the other, and where best developed
are formed of shingle:
They front the bay, and sweep up both
sides of the valley. At Guasco, north of Coquimbo, the
phenomenon is displayed on a much grander scale, so as to
strike with surprise even some of the inhabitants. The terraces
are there much broader, and may be called plains, in
some parts there are six of them, but generally only five;
they run up the valley for thirty-seven miles from the coast.
These step-formed terraces or fringes closely resemble those
in the valley of S. Cruz, and, except in being on a smaller
scale, those great ones along the whole coast-line of Patagonia.
They have undoubtedly been formed by the denuding
power of the sea, during long periods of rest in the
gradual elevation of the continent.
Shells of many existing species not only lie on the surface
of the terraces at Coquimbo (to a height of 250 feet),
but are embedded in a friable calcareous rock, which in some
places is as much as between twenty and thirty feet in
thickness, but is of little extent. These modern beds rest on an
ancient tertiary formation containing shells, apparently all
extinct. Although I examined so many hundred miles of
coast on the Pacific, as well as Atlantic side of the continent,
I found no regular strata containing sea-shells of
recent species, excepting at this place, and at a few points
northward on the road to Guasco. This fact appears to me
highly remarkable; for the explanation generally given by
geologists, of the absence in any district of stratified
fossiliferous deposits of a given period, namely, that the
surface then existed as dry land, is not here applicable; for we
know from the shells strewed on the surface and embedded
in loose sand or mould that the land for thousands of miles
along both coasts has lately been submerged. The explanation,
no doubt, must be sought in the fact, that the whole
southern part of the continent has been for a long time
slowly rising; and therefore that all matter deposited along
shore in shallow water, must have been soon brought up
and slowly exposed to the wearing action of the sea-beach;
and it is only in comparatively shallow water that the greater
number of marine organic beings can flourish, and in such
water it is obviously impossible that strata of any great
thickness can accumulate. To show the vast power of the
wearing action of sea-beaches, we need only appeal to the
great cliffs along the present coast of Patagonia, and to the
escarpments or ancient sea-cliffs at different levels, one
above another, on that same line of coast.
The old underlying tertiary formation at Coquimbo,
appears to be of about the same age with several deposits
on the coast of Chile (of which that of Navedad is the
principal one), and with the great formation of Patagonia.
Both at Navedad and in Patagonia there is evidence, that
since the shells (a list of which has been seen by Professor
E. Forbes) there entombed were living, there has been a
subsidence of several hundred feet, as well as an ensuing
elevation.
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