OF THE EOCKS OF THE SOUTH OF IRELAND. 271 



In all physical aspects, therefore, the resemblance between these 

 groups in Ireland and Scotland is complete *. 



The great hiatus which exists in Ireland between the Old Eed Con- 

 glomerate and the GlengarifFbeds, occurs also in Scotland between the 

 " Upper Old Eed Sandstone" and the " Lower Old Eed." The sections 

 made by the Geological Surveyors in the south of Scotland, show this 

 very clearly. If, therefore, I am right in supposing that this hiatus 

 is filled up by the Middle and Lower Devonians for the Irish area, 

 it holds good equally for the Scotch ; and we are thus able to inter- 

 calate a missing chapter in the interesting physical history of 

 Scotland. 



IS'otwithstanding the almost certain identity in time of the so- 

 called " Lower Old Eed Sandstone " of Scotland with the Gleugariff 

 beds, it is impossible to doubt that it is a formation in the main of 

 lacustrine origin, at least over the Caithness area (" Lake Orcadie " 

 of Geikie). The evidence in favour of this view is too palpable to 

 leave room for doubt, notwithstanding that at times the sea- waters 

 may have made brief incursions. Thus in the district of Tinto and 

 Carmichael, in the south of Scotland, a fossiliferous band is seen in 

 the channel of Carmichael Burn which has yielded fossils of a de- 

 cidedly Upper Silurian character. They consist of Orthoceras clhni- 

 diatum, Ditliyrocaris striata., Graptolites (fragment), and Beyricliia. 

 This band is considered by Professor Geikie to be 5000 feet above 

 the base of the Lower Old Eed Sandstone t; and a band occurs 

 near the base of the formation in the district of Lesmahago, con- 

 taining ^e^r*c7u*« and some obscure remains of Pterygotust. But 

 there is no reason why lacustrine conditions may not have prevailed 

 over the Scottish area while the sea overspread the south and west 

 of Ireland. These lacustrine conditions may also have extended 

 over the north of Ireland, as there is every probability that " the 

 Fintona beds " which occupy so large a tract east of Lough Erne, 

 and which lie unconformably on the Lower Silurian beds of Pomeroy, 

 are representative of the " Lower Old Eed " of Scotland. 



If these views are correct, it follows that the term " Lower Old 

 Eed Sandstone " is synonymous with Uppermost Silurian, and that 

 the beds of that name in Scotland are the equivalents of " the 

 passage-beds " of the Welsh borders, " the Foreland grits " of J^orth 

 Devon, and the " Glengariff beds " of the south of Ireland. On the 

 above grounds I suggest the term " Lacustrine Upper Silurian " in- 

 stead of the name hitherto in use in the Scottish area. 



Evidence from Fish-remains. — 'I must here anticipate an objection 

 which will probably be urged on the score of the evidence from fish- 



* Prof. Geikie admits the resemblance between the Lower Old Eed of Scot- 

 land and the Glengariflf or Dingle beds {I. c. foot-note, p. 347). This view is 

 also stated by Prof. Ralph Tate in his 'Historical Geology' (Weale's series), 

 pp. 78, 79, who also concurs with me in considering the Upper Old Red of 

 Scotland to be the representative of the Old Red Sandstone of Ireland. 



t Mem. Geol. Survey Scot., Explan. of Sheet 23, p. 14. 



I Ibid. p. 13. Silurian fossils have also been found in conglomerate of 

 Habbie's Howe, Pentland Hills, supposed to be derivative, and are described 

 by Mr. R. Etheridge, Jun., Proc. Roy. Pbys. Soc. Edin. 1874. 



