Vol. 58.] | CONSTITUTION OF THE BUNTER PEBBLE-BED. 627 
Cyanite.'—This mineral occurs as the unaltered colourless cores 
of some of the shimmer-aggregates (see below) which are so common. 
The cleavage is distinct, the refractive index high, and some grains 
show in convergent light a negative bisectrix perpendicular to the 
good cleavage (100). 
Compound Grains. 
As might be expected in a deposit of this nature, compound 
grains are very prevalent. They consist of the following :— 
Quartzite, which occurs in more or less rounded grains; it is 
of a fine texture, and may be referred to the quartzite that forms 
the mass of pebbles of which the bed is chiefly composed. 
Chert in rounded grains. 
Felsites in cryptocrystalline masses, sometimes showing micro- 
pegmatitic structure. 
Mica-schist in rounded or oval grains, consisting largely of 
white mica with a parallel arrangement. In some cases a well- 
marked knotted structure is noticeable. Included minerals are 
common, and consist of rutile and tourmaline in fine needles, grains 
of staurolite, as well as many other minerals which, owing to their 
minute size, are difficult to identify. 
Shimmer-Aggregates.*—These aggregates, which consist of 
a felted cryptocrystalline mass of a white micaceous mineral, with 
no parallel structure to speak of, are one of the commonest 
constituents of these sands. They are crowded with inclusions of 
great variety. These aggregates are evidently decomposition- 
products from aluminous silicates, probably from cyanite, andalusite, 
and possibly others. 
Cyanite has been recognized as an unaltered core in some cases, 
while altered andalusite from the Skiddaw Slates presents an exactly 
similar structure. 
Among the inclusions the following may be observed :—zircon 
in its usual form, tourmaline and rutile in fine needles, crystals 
and grains of magnetite, and grains of staurolite and sphene. 
Thin hexagonal and rounded grains of a reddish-brown mineral 
also occur, and in some instances are very plentiful: it is probably 
a garnet, possibly melanite. 7 i 
These shimmer-aggregates are slightly more abundant in the 
south. 
Pinite.’—Cryptocrystalline aggregates of a green micaceous 
mineral, usually not more than 0°30 millimetre across, have been 
noted; but they are rare. 
They include grains of an opaque mineral, probably some ore of 
iron, and from their general characters would seem to be a 
decomposition-product of some mineral, probably cordierite. No 
unaltered cordierite has been met with. 
1 J. J. H: Teall, ‘Brit. Petrogr.’ 1888, pl. xliv, fig. 2. 
2 G. Barrow, Quart. Journ. Geol. Soc. vol. xlix (1893) p. 349 & pl. xvi, fig. 5. 
3 H. A. Miers, Mineral. Mag. vol. xi (1897) p. 283, and A. Lacroix, 
‘Minéralogie de la France’ vol. i (1893-95) pp. 517-18. 
Dex 
