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Chap. VIII.] GENERAL SUCCESSION, NORTHERN HIGHLANDS. 169 
Sutherland Mr. C. Peach detected 
those fossils which enabled me to 
suggest that they belonged to the 
older Silurian rocks (p. 164). (See 
the coloured Frontispiece to the 
volume.) 
The age of the fossiliferous rocks 
being thus definitely settled, it fol- 
lowed that the subjacent red sand- 
stone, grit, and conglomerate must 
form either the base of such Lower 
Silurian, or belong to the remoter 
Cambrian era. Now, from obser- 
vations which I made in 1858, it be- 
came certain that the first of these 
views could not be entertained ; for 
it was clearly demonstrated that the 
lowest beds of the Silurian quartz - 
rock series overlap transgressively or 
unconformably the edges of the sub- 
jacent sandstone and conglomerate f. 
By this phenomenon we are taught 
that an interval elapsed between the 
completion of the pebbly and gritty 
beds that constitute the upper masses 
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t Although Professor Nicol and myself saw 
clearly, in the summer of 1855, the ivfraposition 
of a red conglomerate and sandstone to the 
quartz-rocks and limestones of Assynt (as ob- 
served also by Hugh Miller), we were prevented 
by storms from sufficiently extending our obser- 
vations to ascertain if there was any break be- 
tween these two deposits. Being aware that my 
friend Colonel James, E.E., the Superintendent 
of the Ordnance Survey, was about to explore 
these tracts on a tour of duty in the following 
summer, and knowing his talent as a field-geolo- 
gist, I requested him to look to the relations of 
the beds ; and I have much pleasure in stating 
that I was first made acquainted by him with the 
fact of the unconformability of the younger to 
the older formation along the flank of Suilvein 
(July 1856). Later in that summer, Professor 
Nicol observed the same phenomenon on a larger 
scale, and ascertained that it extended over 
a larger area of the western shores of the 
Highlands. See his memoir ' On the Eed Sand- 
stone and Conglomerate and the superposed 
Quartz-rock, Limestone, and Gneiss of the N.W. 
coast of Scotland,' Quart. Journ. G-eol. Soc. Lond. 
vol. xiii. p. 17 &c, with clear and accurate sec- 
tions. It is right to remark that part of the over- 
lying group here spoken of as gneiss is merely 
one of the Lower Silurian rocks more altered 
than the other strata of quartz, marble, &c, 
and is of a very different character from the 
immeasurably older granitoid Laurentian gneiss, 
which is represented in the accompanying sketch 
and section as forming the buttresses on which 
H ^ +2 « a ^ -= the red sandstones and conglomerates of Cam- 
H ^ 3 o .13 eg brian age are seen to repose. 
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