126 Prof. J. F. Blake—Glaucophane in Anglesey. 
“Tt occurs always in prismatic crystals, bounded by the faces of 
coo P (100) and occasionally with o P & (010) or om P Z- (100), 
but without terminal faces. The usual cleavage is parallel to the 
prism with the amphibole angle (124° 25’—124° 44’), and the blue 
colour in reflected light makes it easily recognizable. Besides the 
cleavage, there is a very characteristic splitting, in which it particu- 
larly resembles actinolite. Glaucophane in the amphibole group 
corresponds pretty nearly with jadeite amongst the pyroxenes. The 
angle of extinction as measured on the plane of symmetry is very 
small, 4°—6°. The pleochroism is particularly clear and fine 
—c= sky-blue to ultramarine, rarely blue-green; 6 = reddish 
violet to bluish violet; a = nearly colourless to golden green. 
8.G. = 3:0—38'1. Glaucophane appears to be entirely confined to 
the crystalline schists and phyllites. The paragenesis of glauco- 
phane is the same as that of actinolite and common hornblende, it 
is associated with diallage, omphacite, garnet, epidote, mica, and 
rutile.” 
The characters are perfectly possessed by the mineral in the 
Anglesey rock. It occurs in narrow elongated prisms, which are 
generally twisted round the epidote grains, and run into each other ; 
but in clear spaces, filled with quartz, some isolated narrow crystals 
appear, at least 8 times as long as broad, and appearing to die out 
like wedges at either end; in other words, they have no terminal 
faces. These prisms seen in polarized light without the analyzer 
are blue when their long axes are parallel to the principal, 7.e. the 
short axis of the Nicol, and nearly colourless when perpendicular. 
In ordinary light these of course produce a lighter tint of blue. As 
the mineral passes over into chlorite, this blue changes into green, 
as it may be seen to do in a single slide. As the long axis of a 
hornblende or glaucophane crystal les near to c, this blue is the 
characteristic one, and the colour seen in the perpendicular direction 
is a combination of the other two. When the prism is cut perpen- 
dicularly to its length, a rhombic section is seen. If the longer 
diameter of this rhombus, which corresponds to 6, is placed parallel 
to the principal plane of the Nicol, the colour is dirty violet; 
and if the short one corresponding to a is so placed, the crystal 
becomes colourless. With regard to the angles of this rhombus, its 
very small size, ‘004 in., in longer diagonal, and the uncertainty of the 
direction in which it may be cut, renders the observation not very close. 
But the acute angle in three distinct trials I estimated at 55°, 56°, 
and 55° 20’, giving a mean of 55° 27’, or for the obtuse angle 
124° 33’, which is within the limits given by Rosenbusch. These 
are the usual forms, but in one case is seen a rhomboid whose obtuse 
angle was estimated at 125°, and which would therefore probably 
be obliquely cut, as shown also by one pair of sides being lengthened. 
and in this the acute angles are cut off by narrow planes making 
equal angles with the two sides. While therefore the rhombuses 
have their edges formed by 110, this shows the trace of the 010. 
I have not observed any crystal with the trace of 100. The lines of 
cleavage are parallel to the faces of the rhombus. With regard to the 
