390 John S. Flett—Orthite in Scottish Rods. 



often zonal, bands of a paler and darker brown running parallel to 

 the edges of the section. Unlike the epidote it usually accompanies, 

 it has a strong tendency to idioraorphisui, and its outlines are straight 

 lines, evidently the traces of crystalline faces. But scattered through 

 the epidote are many little brown flakes of irregular form, feeble 

 pleochroism and strong double refraction, which in all ^^I'obability 

 are to be ascribed to orthite, though they cannot with certainty be 

 determined as such. Two principal forms are to be recognized in 

 the sections of this mineral, one of these being elongated, bounded 

 by two parallel faces and having obtuse terminations. These have 

 ail a straight extinction, their axis of elongation being evidently the 

 orthodiagonal. In convergent light they jdeld the emergence of 

 an axis or bisectrix, and the optic axial plane is transverse to the 

 length of the section or parallel to the plane of symmetry. The 

 other typical section is that perpendicular to this (parallel to the 

 optic axial plane), and in consequence showing neither axis nor 

 bisectrix in convergent light. Its outlines are generally six-sided, 

 though not of symmetrical shape. Its bounding faces seem to be 

 the basal plane (001, M), the orthopinakoid (100, T), and an 

 orthodome (101, r) ; at any rate, the measured angles in the sections 

 giving the most oblique extinction, and hence most nearly per- 

 pendicular to this zone, show a fair approximation to the angles 

 between these faces. Cleavage is either absent or too imperfect 

 to yield any index to the faces present, but the face M is parallel 

 to the principal cleavage of the associated epidote. In the section 

 which gives the most oblique extinction the extinction angle is 34° 

 in the acute angle between the vertical and clino-axes. One crystal 

 is twinned simply, the twin plane being the orthopinakoid. 



In convergent polarized light one section from the orthodiagonal 

 zone yields a bisectrix, emerging in the centre of the field, the 

 broad bar of which is coincident with the length of the section. 

 Hence the optic axial plane is the plane of symmetry. In the 

 diagonal position the apices of the hyperbolEe cannot be grasped 

 by a dry lens, the angle 2 E being apparently great. This 

 bisectrix is positive, but as an oil-immersion lens of 1"3 N-A 

 fails to grasp the axes it must be the obtuse bisectrix, the acute 

 bisectrix being a, and the mineral optically negative. We have, 

 then, for the optic orientation b = b, c : a = + 34°. 



The pleochroism of the mineral, while distinct, cannot be said 

 to be strong, and may be expressed as follows : — 



7, brownish green. 

 /3, yellow brown. 

 a, darker yellow brown. 

 Absorption : 7 7 a 7 /3. 



The refraction and double refraction are both strong, the colours 

 in polarized light being bright, but distinctly lower than those of 

 the epidote in the same slice. 



As already stated, the orthite occurs frequently in the midst of 

 patches of epidote, sometimes in grains of ill-defined shape, more 



