TRANSACTIONS OF SECTION C. 563 
zircons of specific gravity below 4:7. They further differ from ordinary zircon in 
their very feeble, or absence of, birefringence. The crystals are dark brown in 
colour and almost opaque, but after ignition they are bright green and quite 
transparent. 
While some of the crystals consist wholly of zircon of this type, others contain 
an intergrowth of a second kind, which may be present in greater or less amount. 
The latter has a higher specific gravity, and increases in density when ignited ; it 
is optically biaxial with very strong birefringence. A section cut perpendicular to 
the principal axis of such a compound crystal shows, when moved across the micro- 
scope-stage in convergent polarised light, a gradual transition from a biaxial to a 
uniaxial figure, the coloured rings at the same time moving outwards and becominy 
further apart owing to the diminution in the strength of the double refraction, 
which is positive throughout ; finally, when the rings have all moved out of the 
field of view, the black cross also disappears and the corresponding portion of the 
section is optically isotropic. The mean refractive index has about the same value 
in all portions of the section. 
Zircon of the first type has been previously described by Professor A. H. Church 
(1875), and by Dr. S. Stevanovié (1903), and from the researches of these and 
other authors it would seem that there are, at least, three modifications of zircon, 
viz. :— 
a. Those of specific gravity 4:0, which do not increase in density when ignited, 
8. Those of specific gravity 4°7, also not increased in density when ignited, 
y- An unstable form of specific gravity about 4°38, which when ignited is 
increased in density to 4:7. 
That these different kinds are often intergrown in the same crystal is shown by 
the frequent occurrence of zonal structures in zircon, and further by the behaviour 
of the crystals when heated. A crystal consisting of an intergrowth of a-zircon 
and y-zircon will be increased in density on ignition, but not to the higher limit of 
4-7; on the other hand, an intergrowth of @-zircon and y-zircon will reach the 
higher limit when ignited. 
In crystalline form and chemical composition (as far as could be determined by 
qualitative tests), a-zircon and @-zircon are identical, and these appear to be also the 
same for y-zircon. 
8. A Preliminary Description of Three New Minerals and some Curious 
Crystals of Blende from the Lengenbach Quarry, Binnenthal. By 
R. H. Sotty, 1.4. 
The author dealt with three new minerals, whose composition has not yet been 
determined, for two of which he proposed the names Lengenbachite and Marrite. 
He also described crystals of blende having a brilliant grey metallic coating on 
their surfaces and resembling galena or tetrahedrite. 
9. On the Granite from Gready, near Luaxullian, in Cornwall, and its 
Inclusions. By Professor Kart Busz. 
In 1880 J. A. Phillips published in the ‘ Quarterly Journal of the Geological 
Society’ a description of the granite from Gready, in the parish of Luxullian, 
in Cornwall, and the concretionary patches contained in it. This granite he 
describes as sometimes containing dark-coloured patches of irregular shape, firmly 
embedded in the rock, and exhibiting distinct and sharply defined outlines. On 
examination he found them to be composed of the same minerals as the enclosing 
granite, and consequently considered them to be abnormal arrangements of the 
minerals constituting the granite itself, and essentially consisting of a fine-grained 
variety of the granite in which they occur. 
A few years ago I happened to visit the same locality. The granite is still 
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