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. Yet all this gravel has been
transported, and probably rounded, subsequently to the
deposition of the white beds, and long subsequently to the
underlying beds with the tertiary shells.
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