624 . MR. H. H. THOMAS ON THE MINERALOGICAL [ Nov. 1902, 
straight extinction. These seem to be rhombohedral, but as yet I 
have come to no definite conclusion regarding the mineral species to 
which they belong. Negative crystals are common, as also are gas-. 
and fluid-cavities. The inclusions and cavities usually occur along 
definite lines, which are either parallel to the prism, to the pyramid, 
or to both. 
With regard to the distribution of this mineral, it is abundant 
everywhere; but the zircons with the basal plane well-developed 
are more common in the sands north of Uffculm, than from localities 
farther south. 
Ilmenite.—This mineral was probably abundant, for leu- 
coxene is a prevalent constituent, and may replace original black 
grains, of which only a few are left. 
It is opaque, or only slightly transparent, but of a whitish-yellow 
or greenish-brown colour by incident light. It has a granular 
structure, and the grains are usually round. 
Micaceous ilmenite’ occurs in minute flat plates of a | 
bright reddish-brown colour, enclosing rutile-needles arranged 
symmetrically at angles of 60° one with the other, taking on the 
form of sagenite. 
Here it might be well to mention another ore of iron which also 
occurs in some cases, and that is a deep red mineral in thin flakes, 
made up of minute spheres with a radiate structure: Each sphere 
gives a perfect black cross between crossed nicols. It is probably a 
form of hematite. 
‘Tourmaline’ (Pl. XXXIJI, figs. 14-17).— This mineral is 
abundant in all the residues, and occurs in various forms. The most 
plentiful are stumpy crystals, consisting of a prism terminated by 
the simple rhombohedra p {100} and yz {100}. In these the pleo- 
chroism is intense but variable, even in one and the same crystal, the 
dominant colour being brown. The true form of these crystals 
is somewhat obscured by the rounding which the majority have 
undergone ; indeed, some are almost spherical. 
At the southern end of the Pebble-Bed outcrop they reach 0-25 
millimetre in diameter, but are smaller in the north. Another. 
variety occurs, of a rich blue colour, in radial aggregates of 
acicular crystals (P]. XXXII, fig. 8). Inclusions usually consist 
of minute opaque grains, without crystal-outline. 
It is worth noting that the blue tourmaline, which is so 
common in the so-called ‘ Permian Breccias, and also occurs in 
moderate amount in the sands both above and below the Pebble-Bed, 
is extremely rare in the Pebble-Bed itself. Only a grain or two has 
been met with in samples from the southern portion of the outcrop. 
It is, however, more plentiful in the Pebble-Bed sands north of 
Uffculm. 
1 H. Rosenbusch [transl. Iddings], ‘Microscop. Physiogr. of Rock-making 
Minerals’ 4th ed. (1898) p. 166. 
2 J. J. H. Teall, ‘ Brit..Petrogr.’ 1888, pl. xliv, fig. 3. 
