Page 48 - Atlas of Sedimentary Rocks Under The Microscope

93,94, 95 Carbonate rocks
B i o c l a sts

Brachiopods (continued)

93 shows a number of small impunctate brachiopods with
the large pedicle valve and smaller brachial valve
complete. The roughly elliptical fragment in the lower
centre is a section transverse to the length of the fibres
making up the shell wall, and shows a characteristic fine
net-like structure.
Some spiriferoid and pentameroid brachiopod shells
have an inner layer composed of calcite prisms aligned at
right angles to the length of the shell. In this case the outer
fo liated and prismatic layers arc much reduced in thick­
ness. 94 shows two large shell fragments with a thick inner
prismatic layer. One is below the centre of the field of
view, lying parallel to the bottom of the photograph, the
other is near the left-hand edge. The sediment also
contains brachiopod fragments with the more normal,
fo liated structure in a matrix of fine spa rite, probably of
neomorphic origin (see p. 60) containing grey-coloured
crystals of replacement dolomite.
Some pseudopunctate brachiopods possess hollow
spines. 95 shows transverse sections through several
spines. They have a structure similar to the brachiopod
valve with a fo liated inner layer and an occasionally
preserved outer prismatic layer. The section through the
large spine in the upper left of the picture shows part of
the outer prismatic layer preserved. Note how the shape
of the spine gives the foliated layer a concentric structure.
A longitudinal section through a brachiopod spine can be
seen in the upper left-hand corner of 94.

93: Stained acewte peel. /:. ) ·am Limestone. Lower Carbo­
niferous, Rickl011' Quarry. Derbyshire. England: magnific­
ation x 20. PPL.
9tl: Stained acetate peel. £yam Limestone. Lower Carbo­
n i fe r o us, lleadstone Cwtitq�. Derbyshire, En� land; magnif­
ication x 27, PPL.
95: Stained thin section, l:.)•am Limestone, Lower Carbo­
n!j'erous, Rick/ow Quarry, Derbyshire. Enf(land; magnific ­
ation x 28, PPL.
Other brachiopods and brt1chiopod spines are shown in 77.
103. 106, 120, 123 and 183.

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