OLD RED SANDSTONE OF WESTERN EUROPE. 387 
flagstones into two great groups, which seem to be tolerably well defined 
both lithologically and paleontologically. The lower of these, well exposed 
along the east coast, may be called the Wick and Lybster or eastern group ; 
the upper, copiously developed along the north coast from Reay by Thurso 
to the Pentland Frith, may be termed the Thurso and Reay or northern 
group. 
5. Wick and Lybster Flagstones, Eastern or Lower Group.—Returning 
now to the east coast section we observe, as already remarked, that in the 
first ascending section from the basement beds at Ousedale, the red sandy 
and conglomeratic groups pass upward into sandy red and blue flagstones, 
which crop out in the lower reaches of the Langwell and Berriedale Waters. 
Some calcareous beds are associated with them, in one of: which, under the 
ruined castle at the mouth of the last-named river, Dipterus was obtained 
by Mr C. W. Peacu. The Berriedale fault, however, throws this group out, 
so that we need to pass over a space of rather more than two miles until we 
reach the equivalent strata about An Dun. From this latter part of the 
coast a continuous section can be followed as far as the centre of the 
synclinal trough at Latheron, imterrupted by some dislocations, of which 
the only one of importance appears to be that which crosses the coast-line 
about a mile to the south of Janetstown harbour. On the north side of 
that fault a repetition of part of the flagstones probably takes place, accom- 
panied by some crumpling of the beds. But as this occurs near the top 
of the basin it does not seriously interfere with the construction of the 
vertical table. For rather more than two miles beyond the fault the trend 
of the coast-line nearly coincides with the strike of the strata; but from Port 
na Muic to the red sandy group of Ulbster there is a continuous descend- 
ing section, only interrupted by what appear to be unimportant faults. On 
either side of the trough, therefore, the same series of strata is to be found. 
On the south side the thickness of rock exposed above the red sandstones near 
An Dun is about 5500 feet ; on the north side a protraction of the angles taken 
on the shore gives a thickness of 5000 feet. The comparatively slight discre- 
pancy between these two measurements may be due partly to the thinning out 
of the strata, but more probably in the main to the faulting just referred to. 
If then we place the top of the lower group of flags somewhere about the 
centre of the Lybster syncline, we shall have a mass of strata composing this 
_ group to a depth of about 5000 feet. 
| The base of the flagstones passes down, by imperceptible gradations, into 
the red sandy strata of the Langwell group. The thin-bedded red and grey 
sandy shales and flags of An Dun and the Brough, near Wick, may be 
regarded as the passage beds between the two sets of strata. Above this inter- 
