THE OLD RED SANDSTONE OF THE ORKNEYS. 419 



where, and the red matter is uniformly disposed through the rock, except where leached 

 out by percolating water, or where aggregated into irregular layers of iron pan. 



In Sanday, along the west shore, the beds have a very similar character, but are more 

 friable, owing to the admixture of dark red clay. In Shapinshay red beds practically do 

 not occur, the only representatives of the John o' Groats beds being the yellow sandstones 

 and flags ; but on Holland Head red beds again appear, with every peculiarity to be found 

 in those of Eday. Here, again, the beds are mostly massive sandstones, the red shales 

 being of only secondary importance. The total thickness in this section is about 200 

 feet. In Deerness, red beds form the western shore of Newark Bay, and stretch west- 

 wards nearly to the Castle. Here thick coarse sandstones are mixed with green and red 

 marls. The extreme north point of the parish consists of similar rocks, which are let 

 down by a fault running east and west just south of the Mull Head. They have little 

 of the massive uniformity which characterises the beds of Eday, the alternations in the 

 nature of the sediment being comparatively frequent. Red beds form also the cliff above 

 the Scapa Pier, but in the South Isles area, their best exposure is that from Wide wall 

 in South Ronaldshay, by Roeberry, to Hoxa, and thence to St Margaret's Hope along the 

 shore. Here the dip is gentle to north and north-west, and the underlying beds of yellow 

 sandstone pass up very gradually into the deep red marls beneath Roeberry House. 

 The thickness of these marls — which contain thin beds of red sandstone — is considerable, 

 and they resemble closely the beds seen in Calf Sound, in Eday, in every respect, except 

 their greater thickness. Similar beds are to be seen below Smiddybanks in St Margaret's 

 Hope. Overlying these there come in massive coarse red sandstones, which occupy the 

 rest of the area up to Hoxa, where they are faulted against the flags of Hoxa Head. 

 The thickness exposed in this section is about 500 feet, and not greatly less than that of 

 Eday, where the yellow sandstones are so insignificant. The whole thickness of the 

 John o' Groats beds of Orkney may thus be put down at about 1000 feet in its greatest 

 development. Red beds occur also in Burray and Hunda, but these present no features 

 of special interest to merit a separate description. 



With these red sandstones the long history of the Orcadian Old Red of Orkney comes 

 to a close. A complete change in the nature of the sediment accompanied what must 

 have been considerable changes in the physical conditions of the area. Yet it is, after 

 all, only a reversion to that type of deposit which elsewhere had been the main one for 

 vast periods of time. In the nature of its rocks and in the limited development of 

 volcanic activity, this area had long been a great contrast to the Old Red of Southern 

 Scotland ; only at its close do we find a partial resemblance to make its appearance. 

 The red sandstones are the least important part of the Orkney Old Red. Neither in 

 Caithness nor Orkney do we find them conformably overlaid by any other rock. The 

 new conditions which supervened were marked by the precursors of a new fauna, of 

 which the first example is the Asterolepis, a fish so characteristic of the upper Old Red 

 of the southern shores of the Moray Firth. But before that fauna was to attain its 

 greatest development great changes in the physical geography of Scotland had to take 



