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:--