388 PROFESSOR GEIKIE ON THE 
mediate zone, the flagstones become thick-bedded, with intercalated sandstone 
bands, and occasional seams of shale and argillaceous limestone. The prevail- 
ing colour is bluish-grey, though the red hue of the group below still appears 
both as a local superficial stain and as the tint of separate layers. To the 
south of Wick, thick calcareous blue flagstones and hard grey well-bedded 
sandstones have long been extensively quarried for building purposes. In 
lithological character this group of strata is distinguished from that which over- 
lies it by the greater massiveness of its flagstones, and by their less calcareous 
composition and less fissile or shaly texture. But, in general aspect, the two 
groups so agree that they must be regarded as essentially one great flagstone 
series. Their more minute characters will therefore be detailed after the 
position and thickness of the second group have been given, and these details 
will, for the most part, apply to all the flagstone groups. 
Paleontological Features.—The paleontological distinctions of the group are 
in like manner rather vague. I may remark, in reference to this part of my sub- 
ject, that I am mainly indebted for my information regarding the distribution of 
organic remains in Caithness, to the assistance so freely rendered to me by my 
friend Mr C. W. Peacu. Long years of patient collecting in that county made 
him familiar with the best localities for fossils, and he noted what species had 
been met with at each place. Having myself worked out on the ground the 
order of succession of the strata, I was enabled, by means of Mr Pracn’s list of 
localities, to construct the table appended to this memoir, showing the vertical 
range of every species known to occur in Caithness. Many of the same forms 
are also met with in Orkney ; but, as will be afterwards pointed out, their pre- 
cise localities there have not always been noted, so that, in the meantime, no 
corresponding table of ranges can be drawn up for the Orkney region. I have 
placed, however, in Table II., p. 451, the names of all the species hitherto met 
with in Orkney; so that a simple inspection of this table will afford materials 
for a comparison between the ancient ichthyic fauna of Lake Orcadie, as pre- 
served over the areas of Caithness and Orkney respectively. 
In various horizons throughout this group remains of terrestrial plants have 
been found by Mr Peacu, notably at the South Head of Wick, at Kilminster 
(or Kilmster), at Ackergill, at Kiess, and northward in the pale sandstones of 
Freswick, which probably belong to one of the upper groups of Caithness. The 
Kilminster bed, which particularly abounds in these remains, is a grey shale, 
or ‘calmstone,” lying between flagstones, and having its surface covered with 
carbonised vegetation. Among the forms Dawson’s genus Psilophyton pre- 
dominates, both here and throughout the plant-bearing beds of Caithness. Mr 
Peacu thinks there are probably two species. He has obtained some specimens 
showing fructification at Kilmster and at Ackergill. Large sheets of these 
plants, matted together, appear to have been entombed in the mud of Lake 

