Vol. 62.] CATARACTS OF THE RIVER MADEIRA, ETC. 115 



birefringence or relative retardation per unit of length amounts to 

 11 thousandths in the pale portions, and 18 thousandths in the 

 darker. An examination of the figure in convergent light appears 

 to show that the optic axial (binormal) plane is at right-angles to 

 the plane of symmetry and approximately coincident with the 

 plane of twinning ; and that the acute bisectrix is in the plane 

 of symmetry, and is the direction a of vibrations giving the greatest 

 velocity, while c, here the obtuse bisectrix, is the orthodiagonal. 

 The optical sign is, therefore, negative. The section appears to be 

 practically at right-angles to the optic axial plane, but is inclined 

 to the plane of symmetry. The optical characters of the crystals 

 agree with that described by Prof. W. C. Brogger, from Sognsvand 

 (Norway), but differ from most of the others that have been 

 examined, which have the optic axial plane in the plane of 

 symmetry and the optic normal b coinciding with the ortho- 

 diagonal. 1 



Below the pool, about half a mile from the foot of the cataracts, 

 is another reef of crystalline rocks, which has now, however, 

 ceased to interfere seriously with the flow of the river. The 

 greater portion of the rocks that appear above the surface here 

 consist of a fine-grained aggregate of grey quartz and pinkish 

 felspar, with a few grains of black mica visible with a lens (jM 14 ; 

 specific gravity =2* 64). 



In thin sections of the rock (M 14) the microscope shows it to 

 be very similar in character, except in fineness of grain, to the rock 

 of the cataract itself. There is a considerable amount of microcline 

 and orthoclase, the latter having in many cases microperthitic 

 inclusions of albite. Independent albite also occurs. A little 

 biotite is present, showing strong pleochroism. It has dark areas 

 round inclusions of apatite, but these are rather less pleochroic 

 than the rest of the mineral. Small crystals of apatite also occur 

 throughout the rock, and occasionally fluorspar is seen filling the 

 interstices between other minerals ; it is usually colourless, but 

 sometimes shows violet patches. 



At a few points there is a band of crushed material between 

 adjoining crystals, but except for this and an occasional undulose 

 extinction there is no indication of gneissose structure. 



The average diameter of the principal minerals is about a 

 third of a millimetre, but many crystals exceed a millimetre in 

 diameter. 



1 Zeitschr. fur Krystallogr. vol. xvi (1890) special part, p. 95. The character 

 of the movement of the isogyr (the dark brush in convergent polarized light) 

 is sufficient to show that the optic axial plane intersects the section, practically 

 at right-angles, in the direction of the vibration a' ; see F. Becke, ' Die Skio- 

 droinen' Tscherm. Min. & Petr. Mitth. vol. xxiv (1905) pp. 19, v 26. This 

 specimen also appears to agree with the Sognsvand variety, in having the 

 pleochroism C> a >B (namely, with the least absorption in the direction of the 

 optic normal), while the other forms have C>fj > a. 



