460 G. S. Kinahan — Geology of Parts of Gakvay and Mayo. 



Nearly parallel breaks to the south portion of the Glifden Fault 

 have more or less formed the hollows now occtipied by the lakes in 

 the champagne country between Clifden and Roundstone, as most of 

 them have their greatest length in parallel lines. This system of 

 breaks, however, was associated with other systems, the most marked 

 being those that range respectively N. and S., N.W. and S.E,, and 

 N.E. and S. W. ; and with these different bearings, the principal long 

 stretches of the lakes coincide, while every bay, creek, or gut, 

 branching from one of these lake-basins was evidently originally 

 formed along one of these or some minor break. 



The north and south breaks are more conspicuous, and seem to 

 occur more frequently in the country near Eoundstone Bay than 

 elsewhere, but in other places some were noted forming conspicuous 

 features, such as the fault in the south portion of the valley of 

 Lough Inagh, the fault in the pass called Maumv/ee between Shan- 

 naunnafeola and Lackavrea, the fault of the Owenwee valley, and 

 others. 



The system of breaks coinciding with the most marked valleys 

 seems to be that which bears nearly N.W. and S.E., while the prin- 

 cipal ridges among the hills south of Killary Bay have a nearly 

 similar trend. Other conspicuous valleys are those that run along 

 the east and west breaks. 



That the bays, fiords, valleys, and hill-lines are due to the breaks 

 is evident, as they occur in systems. The western portion of Killary 

 Bay, and all the other submerged valleys now forming bays from 

 the Atlantic, southwest of Killary Bay to Mannin Bay, have a 

 general bearing of from N. 55 W. to N. &5 W., being to all intents 

 and purposes parallel, this bearing also coinciding with that of the 

 valley of Lough Muck on the south of the v/estern portion of Killary 

 Bay. Of E. and W. valleys we have the eastern portion of Killary 

 Bay ; Clifden and Oughterard valley, from the first town to Lough- 

 aunerin, and farther eastward from Lough Agnafiard to Lough Corrib ; 

 north of the latter, and of equal length, being the S.K portion of Maum 

 Bay. Another E. and W. feature is the valley from Cleggan Bay 

 through Ballynakill Lake and harbour, and from thence through 

 Kylemore Lake. A few miles further east the valley is crossed by 

 high ground, but the line of break is visible, and still further east 

 on it are situated the picturesque Lough uafoey and Derry Bay. 

 Other E. and W. faults are also conspicuous, forming minor valleys. 

 As before mentioned, the breaks are in systems of different ages, two 

 of which, at le'ast, are older than the intrusion of the oldest granite, 

 as they are cut by veins from this granite ; two, at least, are newer 

 than the granite, but older than the Carboniferous rocks. One is 

 known to be Post-Carboniferous, but Pre-Glacial, while two or three 

 are Post-Crlacial. Some of the N.W. and S.E. faults are very recent; 

 these, however, in places run along lines of much old^r faults. The 

 E. and W. faults would appear also to be very recent, but generally 

 they appear to be displaced by the N.W. and S.E. faults. Some also 

 of the N.E. and S.W. faults seem to be Post- Glacial ; they, however, 

 are evidently displaced by the N.W. and S.E. faults. 



