THE OLD RED SANDSTONE OF THE ORKNEYS. 413 



and very irregular, the sandstone filling up all these irregularities quite unaltered and 

 undisturbed in bedding. In several places the lava is 30 feet thick, but in one little 

 creek its top and bottom surfaces were seen in section, and here it was not over 12 feet 

 in thickness. Its greatest development is to the south and east, from which direction it 

 seems to have flowed from a source now, no doubt, concealed by the sea ; and this con- 

 clusion is strengthened by the occurrence on the same horizon of similar volcanic rocks 

 in the sandstones of Deerness. 



The southern termination of this area of John o' Groats beds corresponds very 

 closely with the shores of Inganess Bay. At more than one place in this district the 

 flagstones have yielded Dipterus macropterus (Traq.) and Tristichopterus alatus (Egert.), 

 and in it occur both types of sediment characteristic of these rocks ; but so completely is 

 it occupied by the sea that little certainty can be attained as to its exact geological 

 structure. Along the eastern shores the rocks are yellow sandstones, with many thin 

 beds of dark brittle flag, dipping mostly N.W. at gentle angles. On its western side, 

 again, the red sandstones and marls of Holland Head are underlain by a fine pure yellow 

 sandstone below Berstane House, which at its lower part contains belts of flagstone, and 

 even an occasional red bed. The proximity to the great fault which runs out to sea in 

 Meil Bay disturbs these rocks somewhat, but there can be no doubt that this is the 

 order of the succession, and that, on the whole, these are higher in the series than 

 the yellow sandstones, which on the other side of the bay rest on the flags of the 

 East Mainland anticline. The area must be somewhat disturbed by faults, for on 

 the shore to the southward, at the west corner of Inganess Bay, we find a patch of red 

 marls which belong undoubtedly to the overlying red series. The yellow sandstones of 

 this area bear a close resemblance to those of Shapinshay, from which they differ chiefly 

 in the absence of any interbedded volcanic rocks. They show also that the Shapinshay 

 rocks are merely the basal part of the series, and that overlying the yellow beds in 

 this area, as in Eday, there is a series of red sandstones and marls of considerable 

 thickness. 



In Deerness occurs an area of John o' Groats beds which in some respects is the most 

 varied and interesting of any in Orkney. Separated from the previous series by the 

 anticline which brings up the lower flags through the parish of St Andrews, it forms in 

 turn a syncline or basin, of which only the northern half is accessible to observation. 

 The axis of this syncline runs probably E.N.E. from Stembuster on the shore south of 

 Dingieshowie, and on the south side of this axis we have only a very short stretch of 

 sandstones along the shore to just south of the Castle of Claisdie, where they pass down 

 into the grey flags of the parish of Holm. Northwards along the shore the dips sweep 

 round, till at Dingieshowie they are E.S.E. ; and E.S.E. and S.E. dips, as already remarked, 

 are far the most prevalent throughout the parish of Deerness. One of the most com- 

 plete and trustworthy sections is that described by me in a previous paper * as stretching 

 from Dingieshowie to Newark Bay along the south shore, but this is in so far incom- 



* Trans. Roy. Phijs. Soc. Edin., vol. xiii. 



