26 Scientific Proceedings, Royal Dublin Society. 



To the north-east, in the vicinity of the Treantagh Presbyterian 

 meeting-house (sheet 10), the old rocks are perpendicular, and 

 strike north-east at the quartzites which lie across their edges. 

 Over two miles further north-east, to the southward of the Leannan 

 river, there are vertical phyllites ; while immediately north of them 

 there are quartzites that dip north-north-west at from 40° to 50°. 

 In the ravine of the Leanaun river, at Milltown Bridge, a little 

 south-east of Kilmacrenan, the unconformability is well marked, 

 the Older Period Rocks dipping south-east, at high angles, while 

 lying on them, and incorporated with their broken edges, is a 

 massive quartzite. The section across this junction is represented 

 in fig. 2, Plate IV. As this junction is exposed only in the river 

 ravine it cannot be examined except while the river is nearly dry. 



In the Xnockybrin district, to the north-eastward of Letter- 

 kenny, the western boundary of the Later Period Rocks consists, for 

 the most part, of fault lines, along which the newer rocks have 

 dropped down against the older ; but between one and two miles 

 north of Letterkenny (sheet 16) there is, on the west of the main fault, 

 a small tract, and a few very small outliers, in which the quartzites 

 were very little displaced, excepting displacements due to intrusions 

 of later traps. In these tracts the quartzites are rolling nearly 

 horizontally across the upturned edges of the Older Period Rocks ; 

 and in the latter, adjoining the small quartzite outlier to the south- 

 west, there are dykes of diorite which can be traced up to the 

 quartzite boundary ; but there they are obliterated on account of 

 the Later Period Rocks having accumulated on them. 



One of the statements put forward as a proof of the non- 

 existence of the unconformability between the Older and Later Rocks 

 is, that the Great quartzite {Lough Salt quartzite) is not a basal 

 group; that in every locality where it occurs in the baronies of 

 Inishowen, Kilmacrenan, and Boylagh, it lies in an inverted 

 synclinal, and that the Cranford sericitic series, with its limestones 

 that seem to lie on the quartzite, is, in reality, the Lough Salt Lime- 

 stone hornhlendite series brought up into this position hy an inverted 

 synclinal. 



This statement will not, however, bear investigation. The 

 Cranford sericitic series, which always lies on the, Grreat quartzite, 

 and the Lough Salt Limestone hornhlendite series, which, in a few 

 places, is found below it, are quite distinct assemblages of rocks. 



