404 MB JOHN S. FLETT ON 



ously north, the flagstones stretching from the Old Head to Halcrow Head, whence a 

 small area of sandstones extends to Windwick. Here a fault brings up the flags with a 

 steep north dip, and at Stews Head these again are overlaid by yellow sandstones which 

 stretch along the shore to St Peter's Church, where again the blue flags are faulted up to 

 form the promontory of Grimness and the north-eastern corner of the isle, and to pass 

 conformably into the yellow sandstones along the shores of Water Sound. 



Of these rocks the lowest are evidently the flags of Brough Head and Old Head in 

 the southern shore, and here, at Banks Geo, with remains of Coccosteus decipiens (Ag.) 

 and of an undetermined osteolepid, I found numerous plates of Coccosteus minor 

 (]\liller), which have been determined by Dr Traquair. The chief importance of this 

 lies in the fact that it establishes the zonal identity of the flags which encircle the sand- 

 stones of Scapa Flow with those which accompany the Eday beds of the North Isles. 

 Here, however, the horizon is, to all appearance, a higher one, as the distance between 

 the Coccosteus minor beds and the sandstones of Halcrow Head is not much over a mile ; 

 and though there is evidence of faulting in the intervening section, it would seem, as 

 stated by Peach and Horne, # that these faults are not of any great magnitude. 



A further interest is lent to the rocks of South Ronaldshay by the occurrence in 

 them of the new species of Asterolepis previously mentioned. Of this I found a plate 

 in a flag quarry on Hest Head. The horizon is that which is, so far as at present known, 

 characteristic of this fish, being in the grey flags about forty feet beneath the base of the 

 Eday sandstones. Another plate of this species was found by Mr Spence of Deerness 

 at the Castle of Claisdie, near Stembuster, in St Andrews, and still another, a year before, 

 by him and myself, a short distance north of Sandside in Deerness. In both these places 

 the geological position is precisely the same ; and it seems, in consequence, to be a fish 

 of very restricted vertical range, and may ultimately prove to be the type fossil of a sub- 

 zone of the Old Red Sandstone of the Orkneys at the top of the Rousay series. That 

 it is to be united with these rather than with the overlying beds is shown by the 

 accompanying fossils, of which the commonest by far is Dipterus valencienesii (Sedgw. 

 and Murch.), which occurs often in very great numbers in this particular belt of rock. 

 Remains of osteolepid fishes also occur, but there is no trace of the distinctive fauna of 

 the Eday sandstones. 



Liihology of the Flagstones. 



When we pass from an examination of their fossil contents to the study of the rocks 

 themselves, at first glance we are apt to be greatly impressed by their monotony, and the 

 endless repetition of beds in no way differing greatly from one another. The effect on 

 Professor Jameson we have already mentioned : his six weeks' journey in Orkney proved 

 the most uninteresting he had ever made. The geologist who is bent on the search for 

 easily recognisable lithological zones which can assist him in the completion of his map 

 is sure to suffer a like disappointment. Immense as is the variety in these beds, no 



* Peach and Hohne, op. cit., p. 11. 



