J. G. Goodchikl — Deutozoio RocJcs of North Britain. 595 



were gradually worn away to the very cores — all before the Upper 

 Old Red Sandstone began to be laid down. 



The full importance of this stupendous unconformity has not yet, 

 I think, l)een adequately realised. When its significance comes to 

 be thorouglily grasped much greater interest than is shown at 

 present will be felt in the geological formations which help to fill 

 in part of the gap. Most of these, as is well known, occur north of 

 the Tweed. 



The newest formation of these Old Eeds, of prior date to the 

 Upper, is the Orcadian Series, which is so extensively developed in 

 the north-east of Scotland and in Orkney and Shetland. The 

 Orcadian Series has formed the subject of several important memoirs, 

 amongst the latest of which may be mentioned those by Messrs. 

 Peach & Home, Sir Archibald Geikie, Mr. Evans, and Dr. Flett. 

 Even the highest beds of this formation lie unconformably below 

 the Upper Old Red ; and neither the natural top of the Orcadian 

 Series nor its base has yet been found. Nevertheless, the thickness 

 of this older subdivision, where it is most fully developed, has been 

 estimated at some sixteen thousand feet. Three well - marked 

 fossilifei'ous horizons occur within it. The highest one occurs in the 

 upper beds, or John o' Groats Flags. This has yielded several 

 fossil fish which have not been met with on any other geological 

 horizon. Amongst these are Tristichopterus alatus, Egert., Dipterus 

 macropterus, Traq., and Microbrachius dicki, Traq. 



Lower down, and on horizons which are near the middle of the 

 series, if we have regard to the maximum development of these rocks, 

 come the fossiliferous beds of Thurso; and, still lower, those of 

 Achanarras and Stromness, as well as at Cromarty and other places 

 near the Moi'ay Firth. Below these, again, come the Sandstones of 

 Berriedale, the Badbea Breccia, and some local conglomerates. These 

 three last-named subdivisions form the lower four thousand feet or 

 so of the Orcadian Series ; but in the absence of any satisfactory 

 palgeontological evidence their position in relation to the highest of 

 the rocks forming the next older series cannot be fixed with 

 certainty. 



From the Thurso Flags Dr. Traquair has identified Homosteus 

 milleri, Coccosteus declpiens, C. minor, Mesacanthus peachi, Cheira- 

 canihus murcMsoni, Diplncanthus striatiis, Bhadinacanthus longispinus, 

 Glyptolepis paucidens, Gyroptychiiis microlepidotus, Osteolepis micro- 

 Jepidota, Thursius macrolepidotus and 27*. phoUdotus, Diplopterus 

 agassizi, and Dipterus valenciennesii. 



The lower or Achanarras subdivision contains a fish fauna which 

 Dr. Traquair has identified as follows : Diplacanthns striatiis and 

 D. tenuistriatus, Rhndinacanthiis longispinus, Mesacanthus pusillus, 

 Cheiracanthus murchisoni and Ch. latus, Fterichthys milleri, Pt. pro- 

 ductus and Pt. oblongus, Dipterus valenciennesii, Glyptolepis paucidens, 

 Gyroptycliius microlepidotus, Osteolepis macrolepidota, Diploterus 

 agassizi, Coccosteus decipiens, Homosteus milleri, and Cheirolepis trailli. 



The fossiliferous portions of the Orcadian Old Eed do not occur 

 to the south of a westerly line drawn through Stonehaven. Whether 



