In descending from these heights I came to a tree which Martinez called copal, but the trunk rose to such an extraordinary height, without branches, that I was unable to make out the appearance of the leaves or flowers. The bark was covered with a milk-white fragrant resin, of a nature analogous to gum thus or gum elemi. The forest also abounds in vegetable and bees' wax, and in many varieties of gums and resins.
On May 11th, as we had now collected a sufficient number of chinchona-plants, including those of the shrub Calisaya which we intended to take up on our return across the pajonales, to fill the Wardian cases at Islay, Mr. Weir began to make up the plants in layers, with plenty of moss between them, ready for sewing up in the Russia matting. Having heard that a young man, a nephew of Gironda's, had planted a C. Calisaya in a small clearing a few leagues up the ravine, I went to examine it. The clearing was on a steep declivity sloping down to the river, and had been partly planted with coffee and coca by its solitary occupant. The tree was a Calisaya morada, having been a root-shoot twelve inches high when it was planted in January, 1859. It is now seven feet high, six inches and four-tenths in circumference round the trunk, and three feet three inches across the longest branches from one side of the stem to the other. It was growing on the side of a steep hill, quite open to the south, east, and south-east, at the edge of a clearing, while mountains covered with forest rise up close behind it, on the north and west, to a great height. It is planted in a soil consisting of stiff yellowish loam, composed of vegetable matter, mixed with the disintegration of the soft clay slate. This is probably the only cultivated chinchona-tree in Peru. In returning to Lenco-huayccu I saw a flock of Alectors, large birds analogous to turkeys, and many parrots; and on my arrival I found that Mr. Weir had already made up the chinchona-plants, in four Russia-matting bundles, ready to start for Sandia on the following morning.
CHAPTER XVI.
GENERAL REMARKS ON THE CHINCHONA-PLANTS OF CARAVAYA.
The range of my observations in the chinchona-forests extended for a distance of forty miles along the western side of the ravine of Tambopata, and one day's journey on the eastern side. This region is covered, with few exceptions, from the banks of the river to the summits of the mountain-peaks, by a dense tropical forest. The formation is everywhere, as I have before said, an unfossiliferous, micaceous, slightly ferruginous, metamorphic clay-slate, with veins of quartz, and the streams all contain more or less gold-dust. When exposed to the weather this clay-slate quickly turns to a sticky yellow mud,[342] and lower down it is very brittle, and easily breaks off in thin layers. The soil formed by the disintegration of the rock, mixed with decayed vegetable matter, is a heavy yellowish brown loam, but there is very little of it on the rocky sides of the ravine, and no depth of soil except on the few level spaces and gentle slopes near the banks of the river. Mr. Forbes, in speaking of the extensive range of Silurian formation, of which the Tambopata hills form a part, attributes the frequent occurrence of veins of auriferous quartz, usually associated with iron pyrites, to the proximity of granite, whence they have been injected into the Silurian slates. In the cooling and solidification of granite the quartz is the last mineral element to crystallize and become solid, and he suggests that, during the cooling, the consequent expansion due to the crystallization of the constituents has forced the quartz and gold, still fluid, into the fissures of the neighbouring rocks, and so formed the auriferous quartz veins. These are only developed in the slate rocks, which, when such veins occur, must be at no great distance from granitic eruptions, either visible, or such as may be inferred to exist.[343]
The chinchona forests which I examined in the Tambopata valley are between lat. 13° and 12° 30´ S. The elevation above the sea, on the banks of the river, is 4200 feet, while the loftiest crests of the mountains which overhang it on either side attain an elevation of about 5000 feet. In the preceding chapter I have given a general idea of the nature of the climate throughout the year, and my stay was too short to enable me to give any more detailed information for most of the months; but I did not fail to take careful observations while I remained in the valley, which will give an accurate idea of the climate during the month of May. During the fourteen first days of May the results were as follows:—
