On the Laurentian Rocks of Donegal and of other parts of Ireland. 249 
the ridge of Brallanmore. Some distance further south, however, in the Fintown 
district the red gneiss is seen in the stream above Mill Bridge almost in contact with 
Lower Silurian quartzite, which descends from the ridge of Scraigs with a strong 
easterly dip.* About a mile still further southwards, the red gneiss is overlaid by 
several hundred feet of the schistose beds, not present in the former locality, before 
the Silurian beds set in. 
Thus it would appear, if the above statements are correct, that the floor upon 
which the Lower Silurian beds were originally deposited consisted sometimes of 
higher, sometimes of lower, members of the older gneissose series, according to the 
extent of the denudation which had taken place antecedently over different portions 
of the whole area. Such conditions would be quite in accord with observations 
over the north-western Highlands of Scotland, where the Cambrian or Silurian beds, 
as the case may be, repose on very different portions of the gneissose series 
according to locality; and this forms one of the points of analogy between the 
geological structure of the two regions. 
(>). As illustrating the second case of unconformity, namely, the obliquity of 
the Lower Silurian beds to the boundary line of the Laurentian, we have the 
clearest evidence of such conditions in the Lough Salt district. This obliquity 
is suggested by the position of the strata as represented on Griffith’s map, and is 
referred to by Mr. Scott as an apparent unconformity. It was the study of 
Griffith’s map which first suggested to my own mind the unconformable relations 
of the two sets of strata, and the probable Laurentian age of the lower series. 
On the 2nd of May, accompanied by Messrs. Symes and Wilkinson, we made a 
careful examination of the rocks lying on either side of the boundary line along the 
Glen River and Lough Salt range. Our first section showing the junction is 
represented in Figure 2, Plate X.X., where in the bed and banks of the stream we 
found laminated gray shales, nearly flat, resting tranquilly upon an old shelving 
floor of granitoid gneiss belonging to the Laurentian series. 
This section was of much interest to us as affording corroborative evidence that 
the junction of the two series of beds is not, in this district, a fault or dislocation. 
Further up the stream at Ardnawark we found beds of white saccharoid marble 
overlying massive reddish granitoid gneiss, and penetrated by pegmatite veins. 
This was the first occasion in which we had seen limestones in the Laurentian 
series. | 
A little further south the structure of the country becomes conspicuous by the 
* Mr. Symes suggests that the boundary may here be a fault, but the evidence is not sufficient to 
enable us to form a definite opinion ; the case is one of an unconformity, or a fault, or possibly, of both , 
combined. ‘ 
+ Mr. Scott states that at Barnesbeg Gap, S. of the L, Salt district the limestone is apparently 
unconformable to the typical granite or gneiss, and full of garnets and idocrase, 
t+ A slight cross-fault seems to occur here, 
