390 PROFESSOR GEIKIE ON THE 
6. Thurso and Reay Flagstones.--The lower group of flagstones, after its 
numerous undulations and fractures to the south of Wick, dips towards the 
north-west at that town, but from Staxi Goe bends round to the north-east 
with low angles as far as the Noss Head, where the beds once more resume 
their north-westerly inclination. After some crumpling and dislocation 
between Castle Girnigo and Ackergill Castle, a steady south-easterly dip 
sets in, which may be traced from Reiss up to Freswick Bay. There is 
thus a syncline in Sinclair’s Bay, of which the axis runs parallel with the 
trend of the coast between Keiss and Freswick : consequently no higher beds 
are anywhere seen here than in the centre of the trough at Ackergill, 
where they probably do not reach to the top of the lower group. Beyond 
this syncline to the north, owing partly to faults and partly to the spread 
of certain upper red sandstones, the passage of the two groups into each 
other is not exposed on the coast. From about Freswick Bay the strata arch 
over again and dip towards north-west and north. They must continue to do 
so steadily, for in a space of four miles the whole of the higher flagstone group 
and the upper portion of the lower must be comprised, yet the greater part of 
this space is occupied at the surface by red and yellow sandstones, belonging, 
probably, to the Upper Old Red Sandstone. 
It is unfortunate that no continuous section can be traced through the whole 
flagstone series. Each of the two groups is admirably exposed on the coast 
sections, but their passage into each other cannot at present be satisfactorily 
recognised. Possibly some of the lower parts of the higher group may occur in 
the centre of the Latheron syncline ; and, on the other hand, a portion of the 
lower group may rise to the surface from below the upper on the north coast 
near Brough. But until a detailed survey of the ground has been made this 
point must remain undecided. 
The interior of the county being for the most part obscured by peat and 
drift, no continuous section can be followed there, though the rocks are exposed 
in a sufficient number of places to allow of the respective limits of the two flag- 
stone groups being traced. A line drawn from the Wart Hill of Duncansbay, 
south-westwards to somewhere about Halkirk, and then westwards by Loch 
Calder to the edge of the crystalline rocks, would approximately mark the posi- 
tion of the passage beds between the two groups. 
To the north of that line the upper group may be traced in many isolated 
inland exposures and in continuous coast sections. A traverse from Broubster, 
near Loch Calder, to Holburn Head, crosses all the lower and central portions 
of the group, but does not reach the top. The coast west from Holburn Head 
trends almost along the strike of the strata, so that no great thickness is there 
to be seen. Eastwards from Holburn Head the shores of the Thurso and | 
Dunnet Bays show a descending series for several thousand feet without reach- 

