408 MR JOHN S. FLETT ON 



contemporaneous volcanic activity. The same conditions prevailed in the Thurso area, 

 as was shown by Sir A. Geikie, the first trace of volcanic rocks being the necks on the 

 shore at Huna, which pierce the red beds of the John o' Groats sandstones.* These 

 physical changes heralded also the appearance of a completely new fauna in the district. 

 It is long since it was shown by the late C. W. Peach that at John o' Groats occurred 

 certain fossils nowhere else to be found, viz., Tristichopterus alatus (Egert.) and 

 Microbracheus Dicki (Traq.),f and to these Dipterus macropterus (Traq.) was subse- 

 quently added J by Dr Traquaili. The same species occur in Orkney, as I have elsewhere 

 shown, and here they form practically the only known fossils of these beds. With the 

 single exception of a specimen of Coccosteus decipiens (Ag.) collected in Newark Bay, 

 Deerncss, by Mr Magnus Spence, and forwarded by him to Dr Traquair, I know of no 

 other fossils which have been found in them. How sudden and complete the change 

 must have been is shown by the following facts. In Eday, Glyptolepis paucidens (Ag.) 

 and Dipterus valencienesii (Sedgw. and Murch.) occur within a few feet of the base of 

 the yellow sandstones. In the Deerness district Asterolepis, sp. no v., Osteolepis macro- 

 lepidotus (Ag.), Dipterus valencienesii (Sedgw. and Murch.), Glyptolepis paucidens [kg.), 

 and Coccosteus decipiens (Ag.) occur in the rocks immediately underlying these beds, 

 Dipterus valencienesii (Sedgw. and Murch.) in some places in vast numbers and curiously 

 small in size. With the single exception already mentioned, not one recurs in the 

 richly fossiliferous flags among the yellow sandstones. It would seem as if these species 

 had been unsuited to the new environment in some manner or other, and their extinction 

 had been rapid and complete. The flags so crowded with remains of Dipterus valen- 

 cienesii, only a few of which have attained their full size, irresistibly impress on the 

 mind the idea of a sudden extermination. At a higher level we find the same confused 

 aggregation of fishes in the flagstone belts among the yellow sandstones, but this is on 

 the horizon of the volcanic rocks, and we shall, probably be right in regarding it as a 

 consequence of the volcanic activity. The rocks of this series, unlike those they overlie, 

 fall perfectly naturally into two main subdivisions, a yellow below and a red above, the 

 latter possibly an index to the change which ensued on a contraction of the area of the 

 old lake, and rendered it the seat of chemical operations resulting in a new type of 

 deposit. 



In their paper on the Old Eed Sandstone of Orkney, Messrs Peach and Horne 

 described with great accuracy the boundaries of these rocks, which they named the 

 ' upper sandstone series ' of the lower Old Red. It will be sufficient if I here give merely 

 a brief account of their distribution. They occur in the centre of the Eday syncline, 

 forming most of the island of Eday and the Red Holm between it and Westray, and 

 lying in a gentle syncline, which is broken by a fault bringing up a strip of flags which 

 stretches from Warness to the Kirk of Skaill. As described by these authors, the 



* Sir A. Gkikie, Old Bed Sandstone, pt. i. p. 405. 

 + British Association Meeting at Aberdeen, 1858. 



X Egerton, Geological Survey Decade. Traquair, Geological Magazine, Nov. 1888. Proc. Roy. I'hys. Soc. Ed 

 1896. 



