458 G. H. Kinahan — Geology of Parts of Galway and Mayo. 



Faults and Dislocations. 



The rocks in this area are cut up and displaced by an innumerable 

 series of faults and dislocations, some being Post-Glacial, while the 

 oldest may be Cambro-Silurian ; and on account of the numerous 

 rock exposures, it is evident that all the features of the country are 

 due to these shrinkage fissures, they having induced the subsequent 

 denudation, — valleys, fissures, and cracks having formed in those 

 places where the "fault-rock" was weak, or the ground broken by 

 dislocations ; while in other places, where the fault-rock was of a 

 harder nature than the associated strata, portions of the dykes form 

 peaks and ridges. The newest faults seem to form the most con- 

 tinuous features, while in some of the older ones the displacement of 

 the rocks is enormous. The latter, however, probably was not accom- 

 plished at one time, but by difi'erent movements in successive ages 

 along the same line. This is quite evident in some places, as will be 

 kereafter mentioned. 



Clifden Fault. — This is one of the most continuous faults. Tt is 

 probably of Post-Glacial age, and can be traced from Dogsbay, three 

 miles west of Eoundstone, north-north-westward, through Lough 

 Fadda, to the town of Clifden, and from thence into Cleggan Bay. 

 This fault displaces everything except the more recent peat ; but 

 along portions of it there is evidence to prove four at least distinct 

 nwvements, one of which occurred prior to the associated rocks being 

 metamorphosed, as in places there are detached portions of a dyke 

 of metamorphosed fault- rock. 



Doolough and Maum Valley Fault. — ^This fault can be traced across 

 the whole country, and probably is very recent. To the N.W. it 

 begins at the Atlantic Ocean, and can be traced through the valley 

 of Doolough to Killary Bay, and from that along the Maum valley, 

 through the N.W. arm of Lough Corrib, and along the valley of the 

 Owenwee, into the Clifden and Oughterard Valley, south of which 

 its position is obscured by deep bog, but it probably extends along 

 a string of lakes into Galway Bay. All the faults that extend 

 southward into the low granite country are more or less obscure on 

 account of deep bog, besides not making such marked features as 

 elsewhere, and probably being of less magnitude, 



Loughinagh Valley Fault. — This does not extend in so straight a 

 line as those previously mentioned. It seems also to split and form 

 two or more lines, but the deep bog in the valley may mislead 

 the observer. To the N.W., in the Kylemore Lough Valley, there 

 is a marked fault that turns S.E. into the Loughinagh Valley, where 

 it seems to split into two great faults, one running S.E. and the 

 other nearly due south. 



Clifden and Oughterard Valley Fault. — This occurs in the valley 

 between Clifden and Oughterard. In some places it would appear to 

 be very recent, and to displace all faults it comes in contact with ; 

 while in others it seems to be displaced. It is, however, very hard 

 to trace, on account of the deep bog and drift in the valley, the latter 

 having been re-arranged during the " Esker-sea or the 350-feet beach 

 period;" also that the fault in general runs nearly with the strike of 



