226 W. II. Hdbbs — Paragenesis of Allanite and 



marked by a beautiful zonal structure absent in the epidote. 

 The forms observed were M (001), r (101), T (100), and some- 

 what imperfect terminal planes, the symbols of which could 

 not be determined. When not too deeply colored the long 



l. 





Sections of intergrowths in the Ilchester granite, x 50. 



sections yield an optic axis, which shows the plane of the axes 

 to be the clino-pinacoicl (010). Such sections therefore ex- 

 tinguish the light parallel and perpendicular to the long axis 

 (b). The index of refraction is strong, while the double refrac- 

 tion is feeble, the interference colors between crossed nicols 

 being of the first order, even when the slides are above the 

 usual thickness. The dispersion is very strong, making indis- 

 tinct the extinctions in polarized light. Twins parallel to the 

 ortho-pinacoid were rarely observed. A chance section of such 

 2. a twin appears in fig. 2. The symmetry of 



extinction with reference to the composition 

 seam and the form of the section, are evi- 

 dence that the plane of the section, lies 

 nearly or quite perpendicular to the axis b. 

 In either individual the axis of maximum 

 elasticity makes an angle of 36° with the 

 vertical axis, but in the absence of cleavage 

 |i and perfect planes, it was impossible to de- 

 * termine whether this axis of elasticity lies 

 in the obtuse or in the acute angle ft. The 

 surrounding epidote is allotriomorphic and 

 1 a single individual, but orientated like one of 



Section of tinned the included crystals, as is shown by its axis 

 allanite crystal with f maximum elasticity, which makes an 

 mantle of epidote, x 5. &ngle of go ^^ the twinnmg pkne of the 



inclusion. The pleochroism of the included mineral is one of 

 its most marked characters, and has been determined to be as 

 follows : a light yellowish-brown, h chestnut-brown, and c dark 

 greenish-brown. The absolution is like epidote c > b > a. 



