But it is in the country about the falls of the Rio Negro that the most curious modification of the seasons occur. Here the regular tropical dry season has almost disappeared, and a constant alternation of showers and sunshine occurs, almost all the year round. In the months of June, July, August, and September, when the Amazonian summer is in all its glory, we have here only a little finer weather about June, and then rain again as much as ever; till, in January or February, when the wet season in the Amazon commences, there is generally here a month or two of fine warm weather. It is then that the river, which has been very slowly falling since July, empties rapidly, and in March is generally at its lowest ebb. In the beginning of April it suddenly begins to rise, and by the end of May has risen twenty feet, and then continues slowly rising till July, when it reaches its highest point, and begins to fall with the Amazon. The district of the greatest quantity of rain, or rather of the greatest number of rainy days, seems to be very limited, extending only from a little below the falls of São Gabriel to Marabitanas at the confines of Brazil, where the Pirapocó and Cocoí mountains, and the Serra of Tunuhy, seem to form a separation from the Venezuela district, where there is a more regular summer in the months of December, January, and February.
The water of the Rio Negro in the month of September did not vary in temperature more than two degrees. I unfortunately lost my thermometers, or had intended making a regular series of observations on the waters of the higher parts of the rivers I ascended.
The extreme variation of the barometer at Pará for three years was only three-tenths of an inch (see diagram, [Plate V.]). The mean height, with all the necessary corrections, would seem to be almost exactly thirty inches; I have, however, already given my reasons for believing that there is a considerable difference in the pressure of the atmosphere in the interior of the country. In the month of May some very cold days are said to occur annually on the Upper Amazon and Rio Negro; but I never myself experienced anything of the kind worthy of notice. Many intelligent persons have assured me that the cold is sometimes so severe that the inhabitants suffer much, and, what is much more extraordinary, the fishes in the rivers die of it. Allowing this to be the fact, I am quite unable to account for it, as it is difficult to conceive that a diminution of temperature of five or ten degrees, which is as much as ever takes place, can produce any effect upon them.
I have an authentic account of hail having once fallen on the Upper Amazon, a remarkable occurrence at a place only three degrees south of the equator, and about two hundred feet above the level of the sea. The children were out at play, and brought it to their parents, astonished at a substance falling from the clouds quite new to them, and which was so remarkably cold. The person who told me was a Portuguese, and his information can be perfectly relied on.
FOOTNOTES:
[4] The sandstone rocks of Montealegre have since been ascertained to be of cretaceous age.
[5] The isolated granite domes and pillars show that the whole area has been formerly covered with thick sedimentary rocks, which have been removed by denudation.
CHAPTER XV
VEGETATION OF THE AMAZON VALLEY.