[[p. 108]] Mr. Wallace, in answer to an invitation by the President, said he had not himself visited the interesting district described in Mr. Chandless' paper. There appeared, however, to be a very singular geographical fact brought out by the discoveries of Mr. Chandless, namely, a very great similarity or parallelism between the tributary rivers on the south of the Amazons and those on the north--particularly between the Purus and the river Uaupés, an [[p. 109]] affluent of the Rio Negro, which he (Mr. Wallace) ascended. It was a very curious circumstance that an immense district of country immediately at the foot of the Andes, both north and south, should, apparently, not receive a single drop of water from those mountains. On the south of the Amazons there was an enormous triangular district, as large as France, between the Madeira and the Ucayali, and immediately below the great range of the Andes, and yet its rivers were not derived from that range. Exactly in the same manner, on the north of the Amazons, the Japura and the rivers east of it appeared to terminate in the great forest-plains before they reached the Andes. He had ascended the Uaupés far enough to ascertain the same fact with regard to this stream. Though prevented from reaching its source, he ascended to a point near a cataract, where the river, though very wide, was a slow, sluggish, black-water stream, and he heard that for 10 days' journey farther up it continued so all the year round. This was a sufficient proof that not a drop of water came from the slopes of the Andes. Hence, there were enormous plains north and south of the Amazons which were, by some means, cut off from the drainage of the Andes. It would be very interesting to ascertain what was the cause of this separation. It would appear probable that it must depend in some manner upon the peculiar contour of the country. There might be a local elevation or ridge near the foot of the range, but separated from it, which caused the water to flow north and south and find an outlet in one of the great rivers. He observed in the map figures indicating the altitude of the river Purus at different points. He wished to ask Mr. Chandless whether those figures could be relied on?

    Mr. Chandless, in reply, said that he believed, quoting from memory from Mr. Wallace's book, that it was found that the barometer stood higher at the town of Barra than at Pará, and he had found that at 600 miles up the Purus it stood higher than at Barra. That, of course, gave a false result as to elevation, but he believed that the observations were quite correct instrumentally. His barometer had been tested at Kew. Some allowance must be made for receding from the equator and the diminution of the equatorial depression of the barometer. He could not believe that at a point 1500 miles from the sea the elevation would be only 107 feet . . .

[[other discussion]]

    Mr. Wallace asked whether Mr. Chandless had any simultaneous observations made at Barra while he was on the Purus.

    Mr. Chandless replied that he had not. He had given, besides the means of his barometrical observations, the assumed means at the sea-level, but could not say whether these were correct . . .

[[other discussion]]