G. H. Ivinahan — Geology of Parts of Galway and Mayo. 459 



the rocks. In places, however, it is very prominent, shifting the 

 transverse faults. This and all other east and west faults are most 

 important, as they cut out thick masses of the rocks; and to them are 

 due some of the greatest complications in the geology of the country. 

 Beginning at the west, we find the fault extending along the valley of 

 the Owenglin, and bringing down on the south of it the rocks belong- 

 ing to Group /S^ (see sections) into juxtaposition with much lower 

 beds. East of Waterloo Bridge to Derrylea Lough, Group ^^ 

 (Great quartzite) and associated rocks are en the north of the fault, 

 while south of it are rocks belonging to Group /3^ (Baltynahinch 

 series). A little east of Derr^dea Lake a cross fault brings higher 

 beds into juxtaposition with the Great quartzite series ; ■ and similar 

 relations occur eastward to the Benglenishj Summit Fault. East of 

 the last fault Group yS* (Ophiolite and Dolomite series), or lower 

 beds, occur on the north of the fault, while south of it are the rocks 

 of Group /3^ Farther eastward to the Lop Bock Fault, which 

 extends northward from the S.W. end of Derryclare Lake, the rela- 

 tions between the rocks north and south of the CHfden and Ough- 

 terard fault are still more complicated. East of the Lop Eock Fault, 

 the Clifden and Oughterard fault may split, as a considerable fault 

 extends along the valley of the Derryclare Lake. This line, how- 

 ever, is a downthrow to the N.W., and brings down Group J3^ 

 against rocks of Group /3^. 



Further eastward the main east and west fault has not such a 

 striking effect, as its throw is not so considerable ; for in the country 

 south of Derryclare, at Lissoughter, and still farther east in the 

 Coreogemore mountains, we find the rocks belonging to the different 

 groups in more regular succession, and with their true dip south- 

 ward; but in places the fault seems to cut out considerable portions 

 of the thickness of Group /@^ or of one of the higher groups. 



Maumina Fault. — This fault is evidently displaced by those pre- 

 viously mentioned, nevertheless it appears to be Post-glacial. To 

 the S.W. it is met with at Mannin Bay, from which it extends nearly 

 E. and W. to Croaghat Lake, being slightly shifted by the Clifden 

 Fault. At Croaghat it joins into a N.E. and S.W. fault. From 

 Croaghat it can be traced, forming a marked feature through Maum- 

 inagh and Gleninagh, into the Lough Inagh Va;lley, where it is 

 slightly shifted by the previously mentioned fault. From this it 

 runs across the hills to Leenaun on Killary Bay, up the upper portion 

 of that fiord, and along the Erriff and Aille river valleys into the 

 Limestone country between Ballinrobe and Castlebar. 



None of the other faults in the district seem to be so continuous, 

 all being greatly shifted by more recent dislocations. Many, how- 

 ever, are very important, on account of the great displacements con- 

 nected with them. Near the west end of Killary Bay, at the pass of 

 Salrock, a fault brings down the Salrock beds against the Doolough 

 beds, and this fault can be traced eastward to Lough Mask, but not 

 continuously, as it is considerably shifted by other faults, the greatest 

 shift being at the Maum Valley — the Maum Valley Fault shifting the 

 eurites on the west of the fault over three miles to the S.E. 



