Prof. J. F. Blake—Glaucophane in Anglesey. 127 
extinctions, when the long prisms are seen, they are either so 
matted together that no direction of edge can be ascertained, or they 
float in quartz, and no good extinction can be observed; but the 
darkest phase makes a large angle of about 15°, with the length 
of the prism. 
4 2 
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Fic. 1.—Prism of glaucophane seen perpendicular to the longer axis ¢. 
Fic. 2.—End of the rhombic prism showing obtuse angle of 124°, 
Fic. 3.—Prism (110) modified by planes of the pinacoid (010). 
Fic. 4.—Showing plane of extinction making angle of 15° with the long axis 
of the prism. 
These observations leave no doubt that the mineral is glaucophane. 
Its associates are epidote, rutile (?), quartz, felspar (?), and calcite. 
The epidote is very abundant in small highly polarizing grains, so 
that the rock is actually a glaucophane-epidote rock. As epidote is 
chiefly characterized by its lime, and glaucophane by its soda, we 
may suppose that the rock is essentially a diorite, in which there 
would normally be a soda-lime felspar and a hornblende; but that 
either at its formation under peculiar circumstances, or by later 
alterations, the soda combined with the hornblendic ingredients to 
produce the variety glaucophane, while the lime caused epidote to be 
substituted for felspar. The rock is singularly free from garnets, 
though one patch may be this mineral. There are also a number of 
small prismatic-looking specks of a rich brown colour, which are some- 
times kneed, and which may possibly be rutile. These are the minerals 
essential to the rock; the others have been produced during the 
squeezings and stretchings to which it has been subject. Their 
interest lies in the fact that some of the crystals of glaucophane float 
freely in the clear crystalline substance, and the untorn substance 
does not look as if it could supply such crystals by tearing. It is 
thus suggested that the glaucophane as such may have been produced 
subsequently to the infiltration of the quartz. 
I have to thank my friend Mr. Teall for suggestions relating to 
this matter, and for taking the trouble to verify the determination of 
the mineral. 
