426 
The highest peaks of the Cordillera in this part of its range do not. 
exceed 6400 feet above the level of the sea. The following section, 
exhibited in the banks of the Santa Cruz in longitude 70° 50’ W., 
is given by Mr. Darwin to illustrate the nature of the plain on which 
the boulders rest. 
: feet. 
1. Gravel, or well-rounded shingle, coarsely stratified, bear- | 
: : : 5 212 
ing chiefly on its surface great angular erratic blocks. . 
9. -Basalticdavar: sez sais tees es Tee ick eee ae arnt 322 
3. Variously coloured thin strata, the lower ones contain- 
ing minute pebbles of the same nature as the boulders, 588 
with the exception of the lava ........ 0... 0000000: 
1122 
Bed of the Santa Cruz, above the level of the sea........ 280 
1402 
The shingle bed (1.) extends uninterruptedly to the coast, where 
it is certainly of submarine origin ; and from the general similarity 
of its nature, Mr. Darwin is of opinion, that it was all accumulated 
under the same circumstances. The contrast in the means of trans- 
port between the deposits (3.) and (1.), the former consisting of fine 
particles and the latter of large pebbles and immense blocks of the 
same rocks with the former, is noticed by Mr. Darwin as an inter- 
esting circumstance. 
The valley of the Santa Cruz widens, on approaching the Cordil- 
lera, into an estuary-like plain, which has an elevation of only 440 
feet; and it is believed by Mr. Darwin to have been submerged 
within the post-pleiocene period, because existing sea-shells were 
found near the mouth of the plain, and because terraces, which, near 
the coast, certainly are of recent submarine origin, extend far up the 
valley. Around this estuary-like plain, and between it and the 
great high plain, is a second plain, 800 feet in height, the surface 
of which, as well as the bed of the river in this part, consists of 
shingle with great boulders. Some of these are of granite, sienite 
and conglomerate, rocks, which were not observed by Mri:Darwin 
on the high plain; and on the contrary, the boulders of basaltic 
lava which were so numerous there, were entirely absent from this 
lower plain and the river-course. From these circumstances, and 
likewise from the immense quantity of solid matter which must have 
been removed in excavating the valley of the Santa Cruz, the author 
infers that the boulders on the intermediate plain and in the bed 
of the river, between 80 and 40 miles from the Cordillera, are not 
derived from the wreck of the high plain, but were transported from 
the Cordillera subsequently to the modelling of the country, and 
within, or not long before, the period of existing shells. 
Mr. Darwin did not observe erratic blocks in any other part of 
Patagonia, but he states, on the authority of Capt. King, that large 
fragments of primary rocks occur on the surface of the great plain 
which terminates at Cape Gregory, in the Strait of Magellan. =~ 
