154 



Proceedings of the Royal Iris/t Acade/))//. 



an upthrust from the north, bringing Llundeilo and Arenig strata 

 to a level with — indeed slightly higher than — the highest beds of 

 the Ludlow which are to be observed in the region.^ On the 

 north side of this break, about half a mile west of Eencraff (1822), 

 a stream lays open a section through fine gray conglomerate and 

 sandstone, black slate and chert, and a sill or dyke of felsite, where it 

 tumbles from the older rocks over the Ludlow band southward. The 

 black slate and chert here yielded me graptolites, by means of which- 

 the beds were proved to be of Lower Llandeilo or Arenig Age. The 

 series, including the sandstones and conglomerates above, presents an 

 unbroken sequence ; and no reason can be adduced, so far as our 

 knowledge extends, against considering the rocks around Derryna- 

 sliggan and eastward towards Leenane on the south side of Killary 

 Harbour, older than Bala. That they are of Llandeilo age is rendered 

 the more probable by the circumstances of rocks which occupy a similar 

 position relatively to the Ludlow, at Salrock. 



The great line of break is traceable westward to this latter place, 

 dipping into the Harbour, so as to leave Ludlow strata forming the 

 shore between Derrynasliggan and Salrock "pass." Here, massive 

 sandstone beds dip away from the line of break ; and with fine and very 

 coarse conglomerates, form the peninsula between the Harbour and 

 Little Killary. Llandeilo fossils have been found on the north and 

 south sides of this promontory. ^ Those on the north side have been 

 described as determinative of Lower Llandeilo ;^ and, the dip of the 

 beds being northward, the fossils on the south side must belong to a 

 much lower zone, unless the beds be inverted (see foot-note). That they 

 are inverted would seem to be probable from the fact that portions of 

 a limestone band and calcareous breccia (such as elsewhere is met 

 with in association with the igneous series) are to be seen at each 

 end of Salrock " pass," that is, near the line of break. These cir- 



^ It should be noted, in view of the vast denudation preceding the deposition of 

 "Wenlock beds, as explained on p. 153, which exposed Llandeilo and Arenig strata 

 in the floor upon which the "Wenlock beds were subsequently laid down, that the 

 Scdrock-Bencraff fault may be of no greater throw than just sufficient to cut out 

 the AYeuloek group and portion of the Ludlow, This, however, would still leave 

 the break of considerable magnitude. 



2 As determined in 189-i at the Geological Survey Office in London. 



3 Mr. McHenry informs me that the fossUs (graptolites) on the south side are 

 indicative of Upper Llandeilo, while those on the north side are Lower. This 

 being the case, the order of the beds is inverted, as they dip northerly. 



^ Explanation of Sheets 83-84, p. 28. 



