Kinahajst and Baily — Report on Rocks, 8j'c. 487 



PAET lll.—{Ahstract.) 



Lithologically the Irish Silurians laay be divided, into coarse accu- 

 mulations ("red arenaceous" type) and^we accumulations (" green and 

 grey argillaceous" type), generally containing marine fossils; the 

 former being the rocks commonly called "Lower Old Eed Sandstone," 

 and by Jukes, "Dingle or Glengariff Grits." This division, how- 

 ever, is no indication of relative age ; for although the rocks of the 

 first type are more often above the others, this is not always the case. 

 On account of the Silurian strata having been deposited in separate 

 basins, the littoral rocks, usually coarse, must be on different horizons, 

 as shown in the plate of vertical sections (Plate XYII.). 



The age of the British rocks called " Lower Old Red Sandstone" 

 has lately been prominently brought forward by Dr. Archibald Greikie, 

 in his Paper " On the Old Eed Sandstone of Western Europe;"^ and 

 as his conclusions intimately concern the Irish rocks, they are now 

 mentioned. 



Geikie considers the " Lower Old Eed Sandstone" of Great Britain 

 to be a part of the same sequence as the typical Silurians, and he sug- 

 gests five " Basins of Deposition": — 



1 . Lake Orcadia Basin. 



2. Lake Caledonia, or the Middle Scottish Basin. 



3. Lake Cheviot Basin. 



4. The Welsh Lake Basin. 



5. Lake of Lome Basin. 



If these are extended into Ireland, the second might be called the 

 Ulster and Connaught Basin, and the fourth the South Ifimster Basin. 

 The last extends westward into "Waterford, Cork, and Kerry ; it is 

 thus briefly mentioned on account of its having to be referred to again 

 in the present inquiry. 



The western extension of the Caledonian Basin is first met with in 

 Ireland, near Cushendun, on the east coast of Antrim, in a small tract 

 of conglomerate, apparently a portion of the shore beds ; but westward 

 and southward the associated rocks are covered up by Mesozoic and 

 Cainozoic rocks ; in addition to which, at Lough Neagh they must be 

 shifted or " heaved" southward by the 1^. and S. faults of that valley. 



In south-east Londonderry, in the neighbourhood of Draperstown 

 and lloneymore, there are red arenaceous rocks of uncertain age, 

 ■which probably may hereafter be found to be outlying portions of this 

 basin ; but further southward, in Tyrone and Fermanagh, the large 

 tract {Fintona district) of "Lower Old Eed Sandstone," appearing 

 from under the Carboniferous rocks S.E. of Pomeroy and ext nding 

 "westward to the Carboniferous rocks of the Erne Yalley, seer ^ un- 



1 Trans, Moy. Soc. Edin,, vol. xxviii. 



