On the Laurentian Rocks of Donegal and of other parts of Ireland. 251 
margin. Thus it is probable that the quartzite of L. Salt Mountain is over 
2,000 feet higher in position than that which comes in contact with the old oneiss 
at the village of Glen, while still higher beds terminate against the gneiss further 
south in the neighbourhood of the Owenbeg and Owenwee rivers. The absence 
of beds of conglomerate at the junction indicates deposition in still and deep waters.* 
(4.) The North-western Boundary Fault.—On looking at Griffith’s Geological 
Map it will be observed that the boundary line between the old gneiss and the 
quartzites and limestones stretches in a 8.W. direction from a point W. of Glen 
to Dunlewy, a distance of 17 miles, in nearly a perfectly straight line. This line 
we found to be a fault of large vertical displacement (Fig. 5, Plate XXI.) The 
evidence at the two points where it was crossed by us is clear and decisive. One 
of these points occurs at the western entrance to Barnesbeg Gap—east of 
Creeslough. 
The section taken through this village—and extending for at least two miles 
from the boundary-fault—lays open a great succession of beds consisting of schists, 
quartzites, and blue limestones, with beds of hornblendic diorite—dipping steadily 
towards the south-east at angles varying from 45°--70°. On approaching the 
boundary of the gneiss, indicated by the sudden rise of the ground into the rugged 
heights of Creagh, the beds approach the vertical position, and are terminated 
by a sharp reversed fold at the margin of the gneiss itself. Here the rock is 
shattered, full of veins and of “ fault-rock.” 
This fault passes by Lackagh Bridge, the base of Croagh, through Loughs 
Agarrowen, and Aganive, and Calaber Bridge to Dunlewy—the exact position 
being determined at this last-named locality by the verticality of the beds laid 
open in the roadside north of the Church. Here, on the one side, we find the 
quartzites, schists, and diorite beds of Errigal; on the other—the beds of gneiss, 
schist, and crystalline marble of the Laurentian series. The direction of the 
boundary changes at Dunlewy Lough, and takes a westerly direction along the 
valley of the Claddy River. This line is also in all probability a fault-boundary, 
meeting the former at an obtuse angle near Dunlewy House. 
The position of this fault is shown in a section taken across Central Donegal 
(Fig. 5, Plate XXI.), which also shows the relations of the Laurentian and Lower 
Silurian beds on the eastern flanks of the Laurentian Gneiss. 
* The nature of the relations between the Lower Silurian and Laurentian beds of the North of Ireland - 
is somewhat similar to that between the Lower Carboniferous and upper part of the Old Red Sandstone 
with the Glengariff beds in the South. In both cases there is an unconformable overlap of the newer 
beds upon a shelving bank of the older. 
TRANS. ROY. DUB, SOC., N.S, VOL. IL. ?) 4d 
