It is highly remarkable, from being composed, to
at least one-tenth part of its bulk, of Infusoria: Professor
Ehrenberg has already ascertained in it thirty oceanic forms. This
bed extends for 500 miles along the coast, and probably for a
considerably greater distance. At Port St. Julian its thickness is
more than 800 feet! These white beds are everywhere capped by a
mass of gravel, forming probably one of the largest beds of shingle
in the world: it certainly extends from near the Rio Colorado to
between 600 and 700 nautical miles southward, at Santa Cruz (a
river a little south of St. Julian) it reaches to the foot of the
Cordillera; half way up the river its thickness is more than 200
feet; it probably everywhere extends to this great chain, whence
the well-rounded pebbles of porphyry have been derived: we may
consider its average breadth as 200 miles, and its average
thickness as about 50 feet. If this great bed of pebbles, without
including the mud necessarily derived from their attrition, was
piled into a mound, it would form a great mountain chain! When we
consider that all these pebbles, countless as the grains of sand in
the desert, have been derived from the slow falling of masses of
rock on the old coast-lines and banks of rivers, and that these
fragments have been dashed into smaller pieces, and that each of
them has since been slowly rolled, rounded, and far transported,
the mind is stupefied in thinking over the long, absolutely
necessary, lapse of years.
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