24 Scientific Proceedings, Royal Dublin Society. 



The Later Period Bocks haye been considerably upthrusted; and 

 as the junction of the Older and Later Period Rocks was a line of 

 weakness, it lying between tbe hard quartzite above and the softer 

 schist below, it may be naturally expected that it would often give 

 way, which is found to have been very generally the case, as the 

 Great quartzite so often was pushed over the unconformability on 

 to the Older Period Rocks, thus obliterating it, so that the majority 

 of the apparent unconformabilities, now to be seen, are due, not to 

 deposition, but to displacement along thrust planes. 



Furthermore, the unconformability has also been obliterated 

 or obscured by other causes, one being the intrusions of whinstone 

 at the junction of the two formations, which in places have cut 

 out or replaced considerable masses, especially of the Later Period 

 Rocks, while miles of the boundary are obscured by accumulations 

 of drift and bog ; and on account of the upthrusts, ordinary faults, 

 the intrusions, and the accumulation of superficial deposits, only a 

 few normal unconformabilities are exposed in the south portion of 

 the barony of Kilmacrenan. 



{For sections of the unconformahilities see Plates I,, II., III., IV.) 



The unconformability between the Lough Salt quartzite and 

 the Older Period Rocks is more or less due to the upthrusting 

 from the south-east ; and in no place is the basal-bed seen, except, 

 perhaps, a mile north-east of Lough Salt, where there seems to be 

 a limestone under the quartzite. This, however, is not quite clear, 

 as the rocks have been disturbed by an intrusion of whinstone. 

 A little south-west of this place, as also a few miles to the north- 

 eastward, small pebbles of the gneissose quartzite are frequent in 

 the Great quartzite. 



To the east of Glen, and further to the north-east, southward 

 of Carrick (sheet 4), the Lough Salt quartzite and the Older Period 

 gneissose quartzite are nearly in contact, and have been classed by 

 the officers of the Survey as belonging to one and the same group 

 of rocks ; but in the latter locality the older rocks are associated 

 with courses of foliated granite, and in the former the gneissose 

 quartzite contains numerous granite veins ; and in neither localities 

 can a trace of the granitic rocks be found in the Lough Salt 

 quartzite. 



The north-western boundary of the Lough Keel and Kilma- 

 crenan quartzite is generally a fault line, which is a downthrow to 



