248 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
to Fintown, where they show a thickness of over a thousand feet. Beds of white 
and blue crystalline limestone may be distinctly seen associated with, not only the 
upper, but the central gneissose series, in the quarry above Glenlehene; also at 
Ardnawark, near L. Salt, at Dunlewy, and Annagarry. The minerals associated 
with these calcareous bands are described by Mr. Scott.* 
It is impossible to estimate with any approach to accuracy the thickness of the 
old gneissose series of Donegal. Even it we assume the gray and reddish granite, 
destitute (or nearly so) of foliation, which occurs at Dunglow, as the lowest portion, 
in no place do we meet with the uppermost beds, as the overlap of the Lower 
Silurian series along the eastern margin conceals them from view. The beds 
generally have a high dip, from 60°-80°, and have a strike parallel to that of the 
great central glen of the Gweebarra and Owencarrow rivers, which traverses the 
wnole gneissose region from Gweebarra Bay on the south-west to Glen on the 
north-east. 3 
(3.) Uneonformities.—There are two forms of unconformity observable between 
the gneissose series and the Lower Silurian beds. If we take the south-eastern 
boundary of the two formations as a datum line, there exists, in fact, an obliquity 
on the part of the gneissose beds on the one hand, and of the Silurian beds on the 
other, to this line.+ 
(a.) As illustrating the nature of the former, it may be observed that different 
portions of the gneissose series are in contact with the boundary line in different 
places. Thus, commencing at the northern extremity of the district at Glen, we 
find the central beds of red gneiss in contact with the Silurian quartzites, &c., as 
may be observed in the Glen river, and at the hamlet of Ardnawark, where the 
Silurian beds repose on massive red gneiss with crystalline white marble. On the 
other hand, south of Crockmore, also near the entrance to Barnesbeg Gap, and around 
L. Acrobane, higher beds of the gneissose series intervene between the Silurian 
boundary and the central beds of red gneiss, consisting of alternating schists and 
beds of white granite, or gneiss, of at least 1,000 feet in total thickness. The 
boundary of the solid red gneiss is well shown by the abrupt rise of the hills west 
of the L. Gartane valley, which is presumably occupied by the softer upper 
schistose beds, and extends to the junction of the Owenwee and Bullaba rivers. 
About a mile up the former of these streams is a fine cascade, where the red massive 
gneiss is well laid open, dipping to the S.E. below the upper series which forms 
* A similar band of limestone occurs in the Laurentian gneiss of L. Maree, in Scotland, and is des- 
cribed by Sir R. I. Murchison, and Professor Geikie in terms which might be applied to the limestones 
of the old gneiss of Donegal. On both sides of the torrent Foulish it occurs as “a snow-white saccharoid 
. marble.”—Quart. Journ. Geol. Soc., Vol. XVIL., p. 177. 
t L admit that the term unconformity is here used in a very uncommon sense, and is applied to the 
outcrops of the surfaces of stratification, and the junction of the two formations. The two phenomena 
in question are independent, and, therefore, corroborative evidences of unconformity in the ordinary 
sense. 
