Vol. 58.] CONSTITUTION OF THE BUNTER PEBBLE-BED., 625 
Quartz.—The grains of this mineral vary considerably, both in 
size and in their degree of rounding. The more angular, however, 
seldom exceed 0°6 millimetre, and the perfectly rounded ones are 
seldom more than 0°5 millimetre in diameter. 
Secondary growths of silica on the grains have not been met with, 
but Dr. Irving’ records it as of common occurrence in the sands 
above the Pebble-Bed. Oe 
Fluid-cavities are common, occurring both in lines and irregularly 
distributed through the grain. Inclusions are numerous, and 
consist of tourmaline, usually blue, and often arranged in 
radiating masses simulating luxullianite; rutile and apatite, 
in fine needles; and what I take to be sillimanite,” in colourless 
fibrous crystals. 
Much of the quartz gives undulose extinction. 
Brookite (Pl. XXXII, fig. 6).°—This mineral is rare in the 
Pebble-Bed of this district, and has been found only as far north as 
Newtor Poppleford. It is associated with anatase. In the Upper 
Bunter* Sands of Ladram Bay, west of Sidmouth, it is seemingly 
more common. 
It occurs in small, tabular, rhombic crystals (0°15 millimetre), 
flattened parallel to the face (100). This face is characteristically 
striated, parallel to the vertical axis. A positive bisectrix emerges 
at right angles to the tabular face. ‘The index of refraction is high, 
and the double retraction strong. The crystals have a pale yellow 
colour with feeble pleochroism. The lustre by incident light may 
be said to be submetallic. 
Staurolite’ (Pl. XXXII, fig. 7)—This mineral is of a yellow 
to amber-red colour when the grains attain much thickness. It 
occurs in angular and rough grains up to 0°5 millimetre in greatest 
diameter, bounded presumably by the cleavage (010) and that 
parallel to the prisms (110). A conchoidal fracture also occurs. 
The index of refraction is high, and the pleochroism distinct, 
ranging from pale to deep yellow or brown. The double refraction 
is strong, and the interference-tints brilliant. Some grains show a 
biaxial figure with a positive bisectrix, and an angle between the 
optic axes varying between 80° and 90°. The plane of the optic 
axes lies in the plane bisecting the acute angle formed by the 
intersection of the prismatic cleavages. 
The grains are mostly very fresh, there being hardly a trace of 
decomposition, Occasionally, however, grains may be seen passing 
into a micaceous decomposition-product. 
1 Rey. A. Irving, Quart. Journ, Geol. Soc. vol. xlviii (1892) p. 71. 
* H. Rosenbusch [transl. Iddings], ‘ Microscop. Physiogr. of Rock-making 
Minerals’ 4th ed. (1898) pl. xvi, fig. 3. 
* W. F. Hume, ‘Chem. & Micro-mineral. Researches on the Upper Cre- 
taceous of S, England ’ 1893, p. 97. 
* E. Hull, Quart. Journ. Geol. Soe. vol. xlviii (1892) p. 65. 
° H. Rosenbusch, op. supra cit. p. 201. 
Q.J.G.8. No. 232. 2x 
