Vol. 58.] | CONSTITUTION OF THE BUNTER PEBBLE-BED. 623 
synthetic (figs. 11 & 12), or more rarely geniculate (fig. 13). The 
long axes of the indrviduals make an angle of about 65° one with 
the other. Inclusions are not common. 
Although this mineral, as has been stated, is common all through 
the Red Rocks, it is more plentiful to the north of Uffculm. 
Cassiterite is difficult to distinguish from rutile, but it 
oceurs in small quantity in the heavy residues from some of these 
sands. There seem to be two modes of occurrence: (i) in-pale- 
yellow, well-shaped crystals, and (ii) in more or less rounded 
brownish grains. 
The crystals are generally short tetragonal prisms, with an 
ordinary pyramid, A basal plane is usually present. The grains 
and crystals differ considerably from the rutile in the same sample. 
The prism seems to be less elongate, the colour is paler, twinning 
is less frequent, and the lustre is submetallic. 
The crystals have been compared with stream-tin from Devon, in 
order to establish their identity. 
This mineral so far has only been obtained from the sands north 
of Uffculm, and does not appear to be present in the sands south 
of that locality. 
Zircon’ (Pl. XXXI, figs. 1-9).—Zircon is one of the most 
abundant minerals in the heavy residues of these sands, and occurs 
generally in colourless, highly refracting prisms, in all essentials 
identical with those so commonly met with. The crystals seldom 
attain a length of more than 0°25 millimetre. 
An interesting form has been discovered by my friend, Mr. J. 
B. Serivenor, in the heavy residue from Fitzhead. It consists of a 
‘capped’ crystal (Pl. XXXI, fig. 3)*, the two individuals being 
connected by their basal planes and in perfect optic continuity. 
This is the only example of the kind that has been met with. 
The basal plane (001) is a rare face on microscopic zircons, but 
it has been noticed to be fairly common on the crystals from 
Burlescombe, Milverton, and Fitzhead. Although this mineral is 
usually colourless, a pink variety occurring in stumpy prisms, capped 
seemingly by a simple pyramid, has been noted from Newton 
Poppleford in Devon (fig. 9). Colourless, and pale-brown stumpy 
crystals also occur. These last-mentioned varieties have all the 
optical properties of zircon, and evidently belong to that group: if, 
however, they are not true zircons, they must belong to an allied 
Species, possibly xenotime.’ 
Many crystals of zircon show a beautiful zonal structure, which is 
very characteristic. 
Inclusions in this mineral are common, and are seen to consist 
of colourless needles, probably apatite, and of a dark-brown, 
highly pleochroic mineral, crystallizing in stumpy prisms, with 
1 J. J. H. Teall, ‘Brit. Petrogr.’ 1888, pl. xliv, fig. 1. 
2 A. B. Dick, ‘ Nature’ vol. xxxvi (1887) p. 91. 
° OQ. A. Derby, Mineral. Mag. vol. xi (1897) p. 308. 
