The tributary Sumidoro, 30 m. wide at its mouth, entered the Arinos from the west-south-west at this point. Its water was deliciously clear. A little way off to the left we could hear the noise of a waterfall on the Sumidoro, before it joined the Arinos.
The river, after the meeting of this important tributary, became even more exquisitely beautiful than before. Rocks strewn about added to the picturesqueness of the landscape as well as to the dangers of navigation, while springs of crystalline water, cool and quite delicious to drink, descended here and there from the banks.
The river had an average width of 60 m. in this part, and was much strewn with broken-up volcanic boulders, especially on the left bank. On the right bank was a beach of immaculate white sand. For 300 m. we went over a great stony place with shallow water. We had to be careful, but all the same many times did we bump with great force and get stuck upon submerged rocks—which we could not see owing to the blinding, glittering refraction of the sun upon the troubled waters.
A tributary 4 m. wide, coming from the north-east, entered the Arinos on the right bank. A great number of rubber trees were to be seen on the right bank, where the forest was luxuriant; but not on the left bank, where the growth of trees was scanty. Carandá or burity or tucuman palms were plentiful along the water's edge near the spot where a small rivulet entered the Arinos on the left bank. Two thousand metres farther down we came upon denuded country, low, and liable to inundation when the river rose. Farther on were campos and open country, with the exception of a thin row of trees immediately along the river. On the left we had luxuriant forest, wonderfully healthy, neat and clean. The stream was there beautiful—60 to 70 m. wide.
When we had gone 10 kils. 800 m. more the entire channel became strewn with rocks and mounds only 1 ft. below the surface of the water, and not unlike parallel small dunes of sand with a deposit of gravel upon them. For 700 m. the river was obstructed and navigation rendered somewhat troublesome.
Where the river turned from bearings magnetic 310° to 360° (due N.) we went over a nasty stony place with a strong corrideira above it, and we were confronted with a rocky barrier almost the entire width across the stream. We kept on the west side, the only way where it was possible to get the canoe through. A little farther another corrideira, stronger than the first, obliged us to find a passage on the east side of the river—which bore upon its bank campos and chapada. Curious mounds of white sand and gravel were visible in the centre of the river, and also near the left bank below the second corrideira; then we came to parallel ridges of white sand and gravel right across the river bottom at an angle of 45° in relation to the general direction of the stream.
Two tributaries, one 3 m. wide on the left bank, the other 4 m. wide on the right side (the latter coming from the north-east), swelled the Arinos from that point. The width of the stream was now increased to 80 m., the water being shallow. The bed of the river was ever changing, and supplied me with constant interest. It was adorned with strangely precise triangles of beautiful white sand exposed through a layer of gravel which covered most of the river bottom.
A thickly-wooded hill range, 150 ft. high and extending from W.S.W. to E.N.E., stood to the north of us. Its slopes, eroded by the water, had caused a landslip, leaving bare vertical red rock for half the height of the hill-range and two much eroded spurs of bright yellow and white earth extending into the stream.
The river at that point turned from north to east. Open country was again on our right after leaving the hill range, and lowlands liable to inundation. Soon afterwards, however, higher land appeared with banks 35 ft. high.
Swarms of small white butterflies played upon the banks on the edge of the water.