Chap. XVI. REMARKS ON CHINCHONA-PLANTS. 267 



CHAPTEK XVI. 



GENERAL REMARKS ON THE CfflNCHONA-PLANTS OF 

 CARAVAYA. 



The range of my observations in the chincliona-forests ex- 

 tended for a distance of forty miles along tlie western side of 

 the ravine of Tambopata, and one day's jonrney 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 ferru- 

 ginous, 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,^ 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 yel- 

 lowish 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, attri- 

 butes 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 conse- 



' Huiicc the name Lenco-huayccu. Lenqui is anything sticky in Quichua, 

 and Imayccu a ravine. 



