108 DR. J. W, EVANS OX THE ROCKS OF THE [Feb. I906, 



abundant microperthite much altered, being greyish by transmitted 

 and yellowish-red by reflected light, and occasional small crystals 

 of orthoclase showing Carlsbad-twinning, as well as much albite 

 which is less altered. The latter contains numerous inclusions 

 of a rodlike substance with extinction parallel to its length, which 

 is the direction of vibration of the slower ray. It has a bire- 

 fringence, or rate of relative retardation, of about 17 thousandths 

 (•017), and is apparently a hydrous mica. 



The quartz appears to be everywhere allotriomorphic, and is in 

 many cases secondary, filling up cracks in other minerals. Biotite 

 is also present, more or less altered into chlorite, and there is a 

 little epidote. Veins of quartz occur in the rock. 



The river now follows a winding course for some 50 or 60 miles 

 through alluvial country, until another range of hills of crystalline 

 rocks is passed at the cataract of Morrinhos, a name which 

 signifies ' small mountains.' 



Here are two rocky reefs, through which the stream forces its 

 way. We landed at the second, and I was able to examine the 

 rock (M9). In hand-specimens it appears to be a fine-grained 

 aggregate of brownish-red felspar. Under the microscope it is 

 seen to be an acid granulite. The constituents have an average 

 diameter of 0*25 mm. Many are well-rounded. Larger crystals 

 occasionally occur. There is abundant plagioclase ; the twinning, 

 however, is in many cases but feebly developed. It is apparently 

 albite. This felspar rarely exceeds the size that I have mentioned, 

 but microcline, which is very common, sometimes occurs in crystals 

 1-5 millimetres in diameter. Microperthite is less frequent, but is 

 occasionally found in comparatively-large crystals. There is much 

 quartz, both in large individuals with irregular boundaries and 

 in small rounded grains often minute, sometimes wedged in between 

 the other crystals, and sometimes occurring as inclusions in them. 

 In a few cases the grains exhibit traces of a crystalline outline, 

 and the extinctions show them to be truly idiomorphic. There 

 is a little green mica, representing biotite that has more or less 

 passed into chlorite, and has an index of relative retardation of 

 only about 8'2 thousandths (-0082). Magnetite has also resulted 

 from the same decomposition. Some small sphenes are visible, as 

 well as pyrites passing into haematite. 



Some 15 miles below Morrinhos is the cataract of Theotonio, 

 where the river passes a broad and lofty ridge in a contracted 

 channel like that at the Salto do Girao. 



There are two successive falls, the total drop being 25 feet. 

 Each appears to correspond to a reef of hard rock that traverses 

 the river. Below the second fall is another reef with a narrow 

 opening, through which the stream flows with a strong current. 

 When the water is low, as at the time of my visit, an expanse of 

 bare rock is exposed to view, and in it the structure is well seen. 



The prevailing rock-type appears megascopically to be a fine- 



