402 PROFESSOR GEIKIE ON THE 
stones of commerce, and Mr PeaAcu informs me that none of the best flagstones 
are found below where it ceases to appear. G'lyptolepis elegans occurs plentifully _ 
along the shore of the Pentland Firth. Mr Peacu has never found this species in 
the Wick flagstone group except at Noss Head, and even there only a few scales 
have been observed. Of Parexas incurvus—a species first described by AGassiz 
from the Lower Old Red Sandstone of Balruddery*—spines have been detected 
by Mr Peacu in two localities near Thurso, but they are extremely rare. They 
are imbedded in a light bluish-grey flagstone, each spine being surrounded with 
a dirty mass of organic débris, in which other fish fragments may be recognised, 
as if they had been vomited by, or had passed undigested through, some larger 
predaceous fish. stheria membranacea (Pacht) crowds the surfaces of the 
shales and flagstones to the east of Thurso, its valves, sometimes still united, — 
appearing as small black glossy membranous shell-like markings. Some of 
these Estheria shales remind one of the leperditia beds in the Calciferous 
Sandstone group near Edinburgh, or of the cyprid shales in parts of the Weald, 
and in the Isle of Wight Tertiary series. 
Plant remains abound in many parts of the Thurso flagstone group. So far 
as yet known not a single true marine plant occurs there ; all the traces hitherto 
noticed point to terrestrial vegetation. In general facies this flora recalls that 
described from the Old Red Sandstone of Gaspé by Dr Dawson, who has him- 
self dwelt upon the similarity. Particularly frequent are flat stems completely 
carbonised, varying up to four or five inches in breadth, and sometimes as many 
feet long. As a rule they show no structure, though traces of a coniferous 
organisation have been detected among them. On the better preserved 
specimens may sometimes be observed a regular calamite-like fluting, but 
without the transverse joints of ordinary calamites. These resemble the 
“corduroy” stems so common at Lerwick in Shetland.  Sigillaroid stems 
likewise occur in the plant-bearing flagstones of Mey. Several lepidoden- 
droid forms may be seen in these strata, as well as at the Hill of Forss, 
and, indeed, generally throughout the northern flagstones up to the top 
of the whole series at John o’ Groat’s. Lycopodites Milleri (Salter) and Lepi- 
dodendron nothum (Unger), both of frequent occurrence, were described by Mr 
Sarrer from these strata. Dr Dawson recognised in Mr Pracu’s collection 
what appeared to be closely allied to, if not identical with, his Canadian species 
Psilophyton princeps. Caulopteris Peachtt occurs in a stratum charged with 
plant remains below the flagstones at Stonegun and elsewhere. 
7. Gil’s Bay Red Sandstone.—The fault which occurs at Scotland Haven 
probably does not displace the rocks to any considerable extent, for, as already 
* “ Vieux Grés Rouge,” p. 120. His specific name was recurvus. 

