may be described as a mica-hornblende-picrite. The remainder of the county
is occupied by strata of Old Red Sandstone age, the greater portion being
grouped with the Middle or Orcadian division of that system, and a small
area on the promontory of Dunnet Head being provisionally placed in the
upper division. By means of the fossil fishes, Dr Traquair has arranged the
Caithness flagstone series in three groups, the Achanarras beds at the
base, the Thurso flagstones in the middle, and the John o' Groats beds at
the top. In the extreme south of the county certain minor subdivisions
appear which probably underlie the lowest fossiliferous beds containing the
Achanarras fauna. These comprise (1) the coarse basement conglomerate, (2)
dull chocolate-red sandstones, shales and clays around Braemore in the
Berriedale Water, (3) the brecciated conglomerate largely composed of
granite detritus seen at Badbea, (4) red sandstones, shales and
conglomeratic bands found in the Berriedale Water and further northwards in
the direction of Strathmore. Morven, the highest hill in Caithness, is
formed of gently inclined sandstones and conglomerates resting on an eroded
platform of quartz-schists and quartz-mica-granulites. The flagstones
yielding the fishes of the lowest division of the Orcadian series appear on
Achanarras Hill about three miles south of Halkirk. The members of the
overlying Thurso group have a wide distribution as they extend along the
shore on either side of Thurso and spread across the county by Castletown
and Halkirk to Sinclairs Bay and Wick. They are thrown into folds which are
traversed by faults some of which run in a north and south direction. They
consist of dark grey and cream-coloured flagstones, sometimes thick-bedded
with grey and blue shales and thin limestones and occasional intercalations
of sandstone. In the north-west of the county the members of the Thurso
group appear to overlap the Achanarras beds and to rest directly on the
platform of crystalline schists. In the extreme north-east there is a
passage upwards into the John o' Groats group [v.04 p.0960] with its
characteristic fishes, the strata consisting of sandstones, flagstones with
thin impure limestones. The rocks of Dunnet Head, which are provisionally
classed with the upper Old Red Sandstone, are composed of red and yellow
sandstones, marls and mudstones. Hitherto no fossils have been obtained
from these beds save some obscure plant-like markings, b