THE OLD RED SANDSTONE OF THE ORKNEYS. 397 



Glyptolepis paucidens (Ag.), and with it Homosteus Milleri (Traq.) is the most abundant 

 fossil, — if we except only the head plates and scattered fragments of Dipterus 

 valencienesii (Sedgw. and Murch.). But the last is quite as common, and probably 

 commoner, in the Orcadian beds, while the two former have certainly their principal 

 seat in the beds now to be described. With these a not unfrequent fossil is the little 

 crustacean Estheria membranacea, which, as at Thurso, sometimes covers the whole 

 surface of slabs of rocks, and is, so far as I know, confined to this zone. Other fishes 

 occur — Cheiracanthus, sp., Osteolepis macrolepidotus (Ag.), Diplopterus Agassizi (Traill), 

 Coccosteus decipiens (Ag.) ; but their principal development seems to have been in a 

 previous time, as they are much more numerous in the lower series. Of the fishes 

 peculiar to this zone, Coccosteus minor (Miller) can hardly be said to be rare, seeing 

 that already we know it in three separate and widely distant localities. It is a very 

 suitable fossil for zonal work, as even its scattered bones are so characteristic as to 

 establish its identity readily. Of the different species of Thursius, only one is as yet 

 known to occur ; and indeed, until a means is discovered for diagnosing these fishes from 

 scattered head plates, bones, or scales, it is unlikely that they will ever be recognised as 

 common fishes in this region of Orkney. The state of preservation requires, in their 

 case, to be much more perfect than holds good as a rule of the fossils of these rocks. 



The North Isles District. 



If we now continue eastwards our section through Orkney from Evie, through Rousay 

 and Egilshay (sect, l), we find that in Eynhallow the east dips which prevail in Evie are 

 repeated, and these beds strike evidently across the narrow Eynhallow Sound into the 

 west side of Rousay. In the latter island the east dips which mark this side of the great 

 West Mainland anticline may be said to prevail throughout, but are everywhere very 

 gentle, and are occasionally subjected to a temporary reversal. The terraced faces of 

 the hills, most marked on the west side, show at a glance the simple structure and the 

 almost horizontal disposition of the beds. Along the western coast, the dips are gentle and 

 frequently changing, being mostly north and north-east in the northern half and south 

 and south-west near Westness, but from Hullion along the south coast to Avalshay the dips 

 are very persistently east, except for a brief space below Trumland House, where a very 

 insignificant anticline occurs. East dips are constant on the shore of Rousay Sound. 

 On the north shore the magnificent range of cliffs from Sacquoy Head to the Knee of 

 Rousay around the whole shore of Saviskail Bay exposes an ideal section, which shows a 

 structure slightly more complicated than that seen on the south side of the island. On 

 Sacquoy Head the dips are east, but on Saviskail Head a small anticline, on the south 

 shore of Saviskail Bay another, and in Scockness a third, throw the rocks into gently 

 undulating folds, whose axis is nearly north and south, without anywhere a dislocation 

 of any importance. The island is thus a geological plateau, out of which the agents of 

 denudation have carved the valleys and modelled the surface features. Its heather-clad 



