VIII 
GEOLOGY OF PATAGONIA 
181 
whence the well-rounded pebbles of porphyry have been 
derived : we may consider its average breadth as 2 00 miles, 
and its average thickness as about 50 feet. If this great bed 
of pebbles, without including the quid 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 prob¬ 
ably rounded, subsequently to the deposition of the white beds, 
and long subsequently to the underlying beds with the tertiary 
shells. 
Everything in this southern continent has been effected on 
a grand scale : the land, from the Rio Plata to Tierra del 
Fuego, a distance of 1200 miles, has been raised in mass (and 
in Patagonia to a height of between 300 and 400 feet), within 
the period of the now existing sea-shells. The old and 
weathered shells left on the surface of the upraised plain still 
partially retain their colours. The uprising movement has 
been interrupted by at least eight long periods of rest, during 
which the sea ate deeply back into the land, forming at 
successive levels the long lines of cliffs or escarpments, which 
separate the different plains as they rise like steps one behind 
the other. The elevatory movement, and the eating-back 
power of the sea during the periods of rest, have been equable 
over long lines of coast; for I was astonished to find that the 
step-like plains stand at nearly corresponding heights at far 
distant points. The lowest plain is 90 feet high ; and the 
highest, which I ascended near the coast, is 950 feet; and of 
this only relics are left in the form of flat gravel-capped hills. 
The upper plain of S. Cruz slopes up to a height of 3000 feet 
at the foot of the Cordillera. I have said that within the 
period of existing sea-shells Patagonia has been upraised 300 
to 400 feet: I may add, that within the period when icebergs 
transported boulders over the upper plain of Santa Cruz, the 
elevation has been at least 1500 feet. Nor has Patagonia been 
