u ;
ii
there is not a single point of eruption; yet in former ages,
as we shall hereafter show, deluges of lava flowed from that
very part. It is in conformity with our hypothesis that this
southern region of repose, is at present suffering from the
inroads of the ocean, as attested by the long line of cliff on
the Patagonian coast. Such we believe to have been the
causes of this singmlar configuration of the land. Nevertheless,
we confess that it at first appears startling, that the
most marked intervals between the heights of the successive
plains should, instead of some great and sudden action of the
subterranean forces, only indicate a longer period of repose.
In explaining the widely-spread bed of gravel, we must first
suppose a great mass of shingle to he collected by the action
of innumerable torrents, and the swell of an open ocean,
at the submarine basis of the Andes, prior to the elevation of
the plains of Patagonia. If such a mass should then be
lifted up, and left exposed during one of the periods of
subterranean repose ; a certain breadth, for instance a mile,
would he washed down, and spread out over the bottom of
the invading waters. (That the sea near the coast can carry
out pebiiles, we may feel sure from the circumstance of
their gradual decrease m size, according to the distance from
the coast-line.)
If this part of the sea should now be elevated, we should
have a bed of gravel, but it would be of less thickness than
in the first mass, both because it is spread over a larger area,
and because it has been much reduced by attrition. This process
being repeated, we might carry beds of gravel, always
decreasing in thickness (as happens in Patagonia) to a considerable
distance from the line of parent rock.* For instance,
on the banks of the St. Cruz at the distance of one hundred
* I t is n e e d le s s to p o in t o u t to th e g e o lo g is t, t h a t th is v iew , i f c o rre c t,
will a c c o u n t, w i th o u t th e n e c e s s ity o f a n y s u d d e n ru sh o f w a te r, fo r th e
g e n e ra l c o v e rin g o f m ix e d sh in g le , so com m o n in m a n y p a rts o f E u ro p e ,
a n d lik ew is e fo r th e o c c u rr e n c e o f w id e ly - e x te n d e d s tr a ta o f c o n g lom e ra te ;
fo r th e su p e rfic ia l b e d s m ig h t, d u r in g a p e rio d o f su b s id e n c e , h e co v e red by
f re sh d e p o s its .
miles above the mouth of the river, the bed of gravel is 212
feet thick, whereas, near the coast, it seldom exceeds 25 or 30
fe e t; the thickness being thus reduced to nearly one-eighth.
I have already stated that the gravel is separated from the
fossiliferous strata by some white beds of a friable substance,
singularly resembling chalk, but which cannot be compared,
as far as I am aware, with any formation in Europe. With
respect to its origin, I may observe that the well-rounded
pebbles all consist of various felspathic porphyries; and that,
from their prolonged attrition, during the successive remodellings
of the whole mass, much sediment must have
been produced. I have already remarked that the white
earthy matter more closely resembles decomposed felspar,
than any other substance. If such is its origin, it would
always, from its lightness, be carried further to seaward than
the pebbles. But as the land was elevated, the beds would
be brought nearer the coast-line, and so become covered by
the fresh masses of gravel which were travelling outwards.
When these white beds were themselves elevated, they
would hold a position intermediate between the gravel and
the common foundation, or the fossiliferous strata. To
explain my meaning more clearly, let us suppose the bottom
of the present sea covered to a certain distance from the
coast-line, with pebbles gradually decreasing in size, and
beyond it by the white sediment. Let the land rise, so that
the beach-line, by the fall of the water, may be carried outwards
; then likewise the gravel, by the same agency as
before, will be transported so much further from the coast,
and will cover the white sediment, and these beds again will
invade the more distant parts of the bottom of the sea. By
this outward progress, the order of superposition must
always be gravel, white sediment, and the fossiliferous
strata.
Such is the history of the changes by which the present
condition of Patagonia has, I believe, been determined.
These changes all result from the assumption of a steady
but very gradual elevation, extending over a wide area, and
’'if
it