G. H. Kiiiahan — Dingle and Glengariff Grits. 349 



differences in the rocks of both the Upper and the Lower Old Eed 

 Sandstone in the various areas or basins. 



Let us now return to the Dingle and Glengariff rocks. Geikie, 

 at page 354, divides up the rocks he classes as the Lower Old 

 Bed Sandstone of Britain, into five great " Basins of Deposit." 

 No. 4 he calls the deposit of " the Welsh Lake " Basin, and on 

 looking at a geological map of Great Britain and Ireland, it is 

 evident that the rocks the subject of this paper are some of those 

 accumulated in the north-west portion of this basin. 



Although Griffith classed the Dingle and Glengariff rocks with the 

 Silurians, while Jukes classed them as Lower Old Red Sandstone, yet 

 both would seem to have agreed that the Dingle beds were equiva- 

 lents of the older portion of the Glengariff grits j 1 or — as stated by 

 Griffith before the Society — the visible Glengariff grits, as a mass, 

 are higher up doubtless than the visible Dingle beds as a mass. 



These observers also agreed that while the Dingle beds are capped 

 unconformably by the "Old Red Sandstone," 2 the Glengariff grits 

 extend upwards conformably into the "Old Red Sandstone," and 

 from thence up to the Coal-measures. The Dingle beds to the 

 westward of the promontory, near Dingle, as indicated in the 

 accompanying diagrammatic section, are at least 10.000 feet thick, but 

 eastward higher beds occur ; and it is even possible, although I 

 believe it improbable, that nearly the whole thickness of the rocks 

 may be under the " Old Red Sandstone " of Slieve Mish near 

 Castlemaine. On the other hand, south of Dingle Bay the Glengariff 

 grits, as far as seen, have been estimated to be of about a similar 

 thickness ; this being due to the upper beds occurring in that area, 

 while the lower 3,000 or 4,000 feet, which in the Dingle Promontory 

 rest on the Marine Silurians, here, although probably present, are 

 not exposed to view. 



It is evident that a nearly east and west fault, having a downthrow 

 to the northward, extends along the valley of Dingle Bay, the throw 

 near Killorglin being about 5000 feet, as here the Coal-measures are 

 brought down nearly, if not quite, into juxtaposition with the Glen- 

 gariff grits. 3 The situation, in about which this Dingle Bay fault 

 occurs, is indicated in the diagrammatic section. In this section, 

 other faults, minor breaks and undulations, known to occur in the 

 country to the south, cannot be represented; the horizontal distances 

 however are nearly correct ; that is, the distances from the centre of 

 Dingle Bay to those of Kenmare River and Bantry Bay. The dis- 

 tance from the south boundary of the " Old Red Sandstone" south 

 of Dingle Bay to the north boundary of the " Old Red Sandstone" 

 north of the Kenmare River is about sixteen miles. In the diagram- 



1 In Memoirs Geol. Survey of Ireland, Expl. Sheet 182, f.n. page 10, Jukes says, 

 "an interpretation " (that of Griffith) " which is theoretically the better of the two." 



2 Here and hereafter in this paper the Upper or Carboniferous Old Red Sandstone 

 will be spoken of as " Old Red Sandstone," while the Lower will be called " Glen- 

 gariff grits " or " Dingle beds." 



3 If we consider the difference in the heights of the ground occupied by the " Old 

 Red Sandstone" and that in which we find the Coal-measures, the latter, in depth 

 at the fault, ought to be lying against Glengariff grits. 



