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ME, J. A. THOMSON OX THE HOENBLEXDIC [Xov. 1908, 



drusy, and minute crystals point into the cavities. Sections of the 

 vein give good facilities for studying the microscopic characters of 

 clinozoisite (fig. 5). It stands out clearly from the somewhat turbid 

 felspar, by reason of its higher refractive power. The elongation is 

 parallel to the 6-axis, as in epidote. Most longitudinal sections 

 show numerous strongly-marked cleavages [parallel to C(OOl)], but 

 these are seldom very close. They are crossed by strong but 

 irregular cracks like those in olivine. Cross-sections parallel to 

 B (010) also show the cleavages, and when the crystal is twinned 



Fig. 5. — Clinozoisite-vein in ampJiibolite from GlendalougJi, showing 

 clinozoisite, Tiomhlende, and spliene. (Magnified 30 diameters.) 



parallel to A (100), as frequently occurs, the cleavage-lines meet at 

 an oblique angle on the twinning-plane (2 /3 = 129° nearly), affording 

 a clear demonstration of the monoclinic nature of the mineral. 

 Longitudinal sections without cleavage-traces also occur, and these 

 must be parallel to C (001), and show twin lamellae. The birefrin- 

 gence is variable even in the same section, but the abnormal prussian- 

 blue tints of clinozoisite predominate over the yellows of iron-poor 

 epidote. There is frequently a gradual passage between the two. 

 A zonal structure is often well marked, the clinozoisite occupying 

 the centre and the epidote the margins ; but there is no constanc}^ 

 in this order, and the mineral composition often varies in patches 

 not zonally distributed. 



The felspar of the vein agrees in specific gravity and refractive 

 indices with albite. A few actinolite-needles penetrate it. The 

 vein has thus the character of a coarse saussurite. 



