102 DE. J. W. EVANS ON THE EOCKS OF THE [Feb. I906, 



small river 1 that enters the Madeira midway down the cataract. 

 As is usually the case, the main hills are on the left bank. 



This is one of the largest and most formidable of the cataracts. 

 For the distance of several miles, the river rushes through rocky 

 channels amidst numerous islets. The rocks consist of gneisses 

 similar in character to those at Misericordia. 2 



I have oniy one specimen (M 5 a) from this cataract. In a hand- 

 specimen, it would be described as a haplite of medium grain, 

 being composed of grey quartz and white felspar, without any 

 noticeable amount of ferruginous minerals. 



Under the microscope, it shows abundant quartz in irregular 

 grains, measuring up to a couple of millimetres in diameter. The 

 smaller quartzes are either idiomorphic, or form rounded blebs, and 

 are often included in the felspars. Microcline is very common, and 

 there is occasionally a little oligoclase. A green hornblende with 

 low extinction- angle, biotite, apatite, and sphene are also found. 



In the dry potholes and other hollows, a ferruginous con- 

 glomerate is observed, containing vein-quartz and decomposed 

 felspathic material. There are also hollows in it left by the 

 rotting-out of small twigs and fragments of wood. 



"We passed the cataracts of Periquitos and Araras 3 without 

 landing. Thence there is a stretch of 50 miles without cataracts, 

 where the Madeira runs through alluvial forest, the soil of which is, 

 as a general rule, raised but little above the river. A short distance 

 below Araras, we landed on a large bank of mud and sand. This 

 had dried after the fall of the river, which was nearly at its 

 lowest, and had split into cracks, which were subsequently filled 

 with blown sand. It would be interesting to know whether the 

 phenomenon has been observed in older strata. Cases of similar 

 cracks being filled with aqueous deposits have been frequently 

 reported. 4 



About 30 miles below Araras one reaches the mouth of the 

 Abuna, flowing in from the west-south-west ; and the Madeira, 

 which has hitherto held an almost northerly course, now turns to 

 the east-north-east, as if in continuation of the Abuna, and crosses 

 the crystalline ridges nearly at right-angles, instead of obliquely as 

 before. It is this bend which is repeated by the other tributaries 



1 The sand of this stream is said to be auriferous. 



2 In ' Viagem ao Eedor do Brasil ' vol. ii (1881) p. 283, Fonseca describes the 

 upper portion as formed of great pavements covered with blocks of diorite, 

 some loose, others forming dykes ; there were also cauldrons and elliptical 

 holes such as have been already described. 



3 F. Keller-Leuzinger (' Tom Amazonas & Madeira ' 1874, p. 44) states that 

 the rock at this cataract is a ' lustrous black-green gneiss.' 



4 According to Col. Church (' The Route to Bolivia via the River Amazon ' 

 1877, p. 187), ' about 7 miles down stream from Araras, and abutting on the 

 river, are two singular bluffs, perhaps 100 feet high each, and perpendicular. 

 They appear like isolated hills which have been sawn across by the river, and 

 are of the stiff red-clay formation common to the whole of Brazil, and especially 

 to the Amazon Valley.' I did not notice these, but we travelled a considerable 

 distance above the Abuna before daybreak. Undoubtedly the banks were 

 unusually high at this part of the river. 



