180 S. CRUZ, PATAGONIA. [cuap. 1x. 
sign ofa change. The drifted trunk of a tree, or a boulder of 
primitive rock, was hailed with joy, as if we had seen a forest 
growing on the flanks of the Cordillera. The top, however, of a 
heavy bank of clouds, which remained almost constantly in one 
position, was the most promising sign, and eventually turned out 
atrue harbinger. At first the clouds were mistaken for the moun- 
tains themselves, instead of the masses of vapour condensed by 
their icy summits. 
April 26th.— We this day met with a marked change in the 
geological structure of the plains. From the first starting I had 
carefully examined the gravel in the river, and for the two last days 
had noticed the presence of a few small pebbles of a very cellular 
basalt. These gradually increased in number and in size, but 
none were as large asa man’s head. This morning, however, 
pebbles of the same rock, but more compact, suddenly became 
abundant, and in the course of half an hour we saw, at the dis- 
tance of five or six miles, the angular edge of a great basaltic 
platform. When we arrived at its base we found the stream 
bubbling among the fallen blocks. For the next twenty-eight 
miles the river-course was encumbered with these basaltic masses. 
Above that limit immense fragments of primitive rocks, derived 
from the surrounding boulder-formation, were equally numerous. 
None of the fragments of any considerable size had been washed 
more than three or four miles down the river below their parent- 
source: considering the singular rapidity of the great body of 
water in the Santa Cruz, and that no still reaches occur in any 
part, this example is a most striking one, of the inefficiency of 
rivers in transporting even moderately-sized fragments. 
The basalt is only lava, which has flowed beneath the sea; but 
the eruptions must have been on the grandest scale. At the 
point where we first met this formation it was 120 feet in thick- 
ness; following up the river course, the surface imperceptibly 
rose and the mass became thicker, so that at forty miles above 
the first station it was 320 feet thick. What the thickness may 
be close to the Cordillera, I have no means of knowing, but the 
platform there attains a height of about three thousand feet 
above the level of the sea: we must therefore look to the moun- 
tains of that great chain for its source ; and worthy of such a source 
are streams, that have flowed over the gently inclined bed of the 
