250 On the Laurentian Rocks of Donegal and of other parts of Ireland. 
tread of the hills and valleys. Looking towards the south, we see the quartzite 
escarpment of L. Salt Mountain bending more and more towards the east ; 
while on looking eastward we see the escarpment of Crockmore, consisting of 
alternating beds of gneiss and schist belonging to the Laurentian series, ranging 
very much in the same line of strike. It became evident to us that the two groups 
here impinge one against the other ; and we determined to devote the greatest care 
to the determination of the nature of the boundary line between them. 
The succession of the Lower Silurian series of the Lough Salt* ridge is indicated 
in Figure 3, Plate XX., which is also intended to show the nature of the junction 
between the two groups. The summit of the ridge consists of quartzite, below which 
there is a series of beds consisting of schists, limestones, quartzites, and hornblendic 
rocks of over 1,000 feet in thickness, 
Below these are also other beds of quartzite, &ce.—which crop out in the direc- 
tion of Glen—but are concealed at this spot by the overlap of the upper beds 
along the shelving Laurentian bank. This will be better understood by a reference 
to Figure 4, Plate XX., which is drawn to correspond with the section (Fig. 3, Plate 
On reaching the deep depression of L. Salt, we considered it desirable to fix 
upon a special bed, and trace it westwards, so as to observe the nature of its con- 
tact with the gneissose series. For this purpose we selected a bed of blue, lami- 
nated, limestone (No. 2) about 100 feet in thickness, which forms an escarpment 
overlooking L. Greenan. Following along this bed for a few hundred yards 
south of the lough—and carefully hammering every projecting piece of rock—we, 
at a certain point, lost the limestone, and found ourselves amongst a series of 
strong micaceous schists which we presently recognised as belonging to the 
eneissose or Laurentian series, as shown at the entrance to Barnesbeg Gap, only a 
short distance further south. Crossing the strike in a southerly direction, we 
found the succeeding beds of quartzite, hornblende rock and limestone, successively 
terminating at the margin of the same beds of schist. Then the obliquity of the 
two series became evident; and, as we satisfied ourselves by observations made 
close to the junction that there is no evidence of disturbance or of fracture, we 
concluded that there is here one of the most remarkable cases of unconformable 
overlap to be found in any district of the North of Ireland.t 
From the evidence obtained in the district of Glen and L. Salt, as well as 
at other points along the S.E. margin of the two formations, we came to the 
conclusion ;—that the Lower Silurian strata had been deposited against the 
flanks of a shelving bank of the old Laurentian rocks while it was being depressed 
“beneath the waters of the sea. Subsequent disturbances and denudation have 
brought to light successive beds of various geological horizons along the Laurentian 
* This, as I was informed by the Rev. Dr. Allman, Rector of Kilmacrennan, is a corruption for L. Alt. 
+ The section in the Glen River (Fig. 2, Plate XX.) also shows that the boundary is not a fault. 
