ON OLD RED SANDSTONE. 255 
The pebbles in the caleareous conglomerate consist of quartzite, vein-quartz, and 
jasper. The majority of the pebbles are rounded and every grade from 12 cms. 
diameter downwards has been noted. A few of the pebbles have a pronounced 
dreikanter aspect. There is no definite arrangement of the pebbles in the band, and 
the deposit is not stratified. 
Petrology.—In thin section, the quartz grains are mainly of two types, though 
every intermediate form occurs. The larger (0°80 mm.) are well rounded, with 
abundant inclusions, and show strain polarisation. The smaller (0-08 mm.) are 
angular to sub-angular, with fewer inclusions. Grains of vein-quartz, with inclusions 
and showing strain polarisation are common. The inclusions are generally arranged 
irregularly (sometimes linearly) and consist of rounded and indeterminate bodies. 
Amongst the recognisable inclusions may be mentioned apatite and chlorite (kindly 
identified by Prof. P. G. H. Boswell), which latter occurs in curved green pipes. Small 
cavities and negative crystals have both been recognised in the quartz grains. 
Very little felspar is present in these rocks, but orthoclase, microcline, and acidic 
plagioclase have been determined. 
Rounded grains of a quartz-schist and a fine-grained chert are of frequent occur- 
rence. No organic remains have been recognised in the chert. Hematite occurs as 
a general stain throughout the rock and also as individual grains. Green and white 
mica and chloritised biotite all occur as twisted laths amongst the more resistant 
quartz grains. 
In the coarser grades granular quartz, and in the finer grades calcite (more or less 
dolomitised) form the chief cementing materials. Scattered grains of calcite and 
dolomite are both fairly common in all varieties. 
Re-crystallised calcite plates form the matrix of the calcareous conglomerate ; 
no traces of organic remains have been found in this matrix. 
Mineralogy.—The usual methods of separating the ‘ heavy minerals’ by means 
of bromoform were used, though a modified form of glass separating funnel has been 
employed. The chief characteristics of the minerals obtained are here briefly noted 
in order of their relative abundance. 
Apatite is very abundant at all horizons. The grains are large (0-1 mm.) and 
generally well-rounded, though prismatic forms occur. They are often full of inclu- 
sions giving the mineral a cloudy effect. Sometimes these inclusions are congregated 
in the centre of the grain, which then exhibits a cloudy core. 
Leucoxene (0:08 mm.) is abundant, and shows the characteristic pitted surface in 
reflected light. 
Zircon occurs both as rounded (0-06 mm.) and prismatic grains; some of the 
latter show well-marked pyramidal terminations with sharp crystal edges. Traces 
of longitudinal cleavage can often be observed, whilst other grains exhibit a system 
of curving cracks. Most of the zircons are colourless, though a few purple-coloured 
grains (generally non-ovoid in shape) have been found. Inclusions are abundant, 
some being needle-shaped, others rounded, and others were determined as negative 
erystals. Tourmaline generally occurs as prismatic grains (0°09 mm.), though a 
few rounded forms have been observed. All the grains of this mineral are intensely 
pleochroic, yellowish-brown to a dark greenish-brown, pinkish-brown to black, and 
colourless to green being the chief types noted. Blue tourmaline, only slightly 
pleochroic, occurs sparingly. 
Rutile occurs in two forms ; either yellow prismatic grains (0-1 mm.), pleochroic 
in various shades of yellow, or rounded pleochroic foxy-red grains showing well-marked 
cleavage lines crossing at 60°. 
Garnet is only found at a few horizons. Most of the grains are rounded (0-1 mm.), 
pale pink in colour, and do not show strain polarisation. Some of the grains show 
marked re-entrant angles and cleavages crossing at 70°. 
Green mica, muscovite, pyrites, ilmenite, and magnetite have also been noted. 
Paleontology.—¥our definite fossiliferous horizons have been determined, and the 
Same type of sediment characterises each level. The rock is a coarse quartzose 
sandstone with rounded pebbles of quartz, vein-quartz, jasper and pellets of a fine- 
grained sandy micaceous shale. The fossiliferous bed at St. Monica’s, Durdham 
Down, is 4 feet thick, and occurs at a vertical distance of 27 feet below the base of 
the Carboniferous. In the railway section north of the Durdham Down tunnel the 
fossiliferous levels are 9, 2 and 6 feet thick, and occur at distances of 102, 115 and 
152 feet respectively below the Carboniferous. Remains of three species—Holoptychius 
nobilissimus, Ag., Glyptopomus kinnairdi, Huxley, and Bothriolepis cf. hydrophila, Ag., 
neye been found, and notes on these have been published in the ‘ St. Monica’s ’ paper 
(ibid.). 
