method of certain
geographers of filling up the space with all kinds of shadings in order
to make it look pretty.

We had gone that day 46 kil., the current having been very strong all the
time, which made up for the loss of time in dealing with the many rapids.

I took altitude observations with the hypsometrical apparatus, water
boiling at that spot at 210 deg. 31/2, the temperature of the air being 78 deg. F.
The minimum temperature during the night of July 30th was 63 deg. F.

We began our journey the next morning by going down a bad rapid and
across an awe-inspiring whirlpool. There again we had to lead the empty
canoe down with ropes, and even so we had difficulty in getting her
through safely.

We were in a region of immense volcanic domes composed of eroded rock,
with many small perforations and large holes 2 or 3 ft. in diameter,
highly polished inside as if they had been varnished. Those rocks were
similar to those I had met in the higher part of the plateau of Matto
Grosso. Some of the holes in the rock had been enlarged to a great extent
by the water of the river revolving inside them.

After we had passed the great whirlpool we found many curious mounds of
sand 20 ft. high among great masses of lava and eruptive rock. Those
mounds were formed by musical sand such as we had met before. We called
it in this particular place "moaning sand," as instead of whistling as
usual it produced a wailing sound like the cry of a hungry puppy.

On the right side of the stream we came to an important tributary 50 m.
wide; 500 m. lower down another strong rapid was reached. I was greatly
amused to see how my men now invariably pulled up the canoe when we
perceived a rapid and unloaded her, carefully taking her down with ropes.
One man, nevertheless, had to be in the canoe to steer her. Alcides
always volunteered to take the job.

[Illustration: Interesting Geological Formation below the Salto Augusto.]

Two hundred metres below this rapid the river turned a sharp corner.
There we met one of the most dangerous rapids and whirlpool I have ever
seen. It was quite awe-inspiring to look at those vicious waters, the
water in the vortex being raised like a wall two or three feet higher
than the greatest waves of the rapid by the force with which it revolved.
The entire river--which, as we have seen, was in some places 2,000 m.
wide--was now squeezed through a narrow cut in the hill range, two great
arms of water joining d