Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 101 



probably the same composition. This view is supported by the fact 

 that there are many felspars in which the repeated twinning is only 

 just perceptible. Many of the plagioclases contain approximately- 

 rectangular inclusions, arranged parallel with crystallographie 

 directions of the host. These have a low refractive index, and are 

 apparently isotropic ; they probably represent spaces formed by the 

 action of solvents, and filled with liquid. Xo mierocline is visible. 

 The small quartz-grains are enclosed in the felspar, and are often 

 idiomorphic. There are also a number of minute, more or less 

 rounded inclusions that are arranged in lines passing through all 

 the constituents of the rock. Flakes of biotite more or less con- 

 verted into chlorite are present in considerable numbers. 



Veins of quartz are observed in the rocks of this cataract ; and 

 pebbles of the same material are bound together by ferruginous 

 cement, to form a hard conglomerate that is met with here and 

 there in the alluvium. 



The rocks are, as a rule, distinctly foliated on a large scale. The 

 strike is very variable. The band of crystalline rocks rises to low 

 elevations on either side of the river ; and it is said that a day's 

 journev into the forest to the south-east there are considerable 

 hills. 



The river now runs for 10 miles through low alluvial land, until 

 at the cataract or rapid of Misericordia, another band of 

 crystalline rocks is passed. I landed on the left bank, a short 

 distance above the cataract, and found the gneiss stretching into 

 the forest in low rounded knolls, which may have been worn by the 

 river when it flowed at a rather higher level. 



To the naked eye, the rock (11 5) appears to be a gneiss, consist- 

 ing of an aggregate of small grains of pink felspar, occasional quartz, 

 and bands of a dark-green material. It has a specific gravity 

 of 2-63. The strike of the foliation is north-north-westerly and 

 south-south-easterly, but it is apparently somewhat variable. 



Under the microscope, abundant mierocline is seen, up to a 

 diameter of 2 or 3 millimetres ; there is also some microperthite, 

 as well as albite-oligoclase containing minute inclusions of a colour- 

 less material, similar to that seen in the mierocline of the haplite 

 from the Madeira Cataract : in this case they probably represent 

 a hydrous soda-mica. There are large crystals of quartz, of about 

 the same size as the felspars, and separated from them by a ragged 

 irregular boundary that corresponds to the crystalline outlines of 

 neither : while smaller blebs or rounded crystals of quartz, such as 

 have been already described, occur as inclusions in the felspars. In 

 the greater portion of the rock, the ferromagnesian minerals are 

 present only in small amount, being represented by flakes of mica 

 and hornblendes which elsewhere form the coloured bands. A few 

 crystals of sphene are also visible. v 



After passing Misericordia, a line of hills is seen in the distance, 

 which are passed in the cataract of Tlibeirao, so called from a 



