CHAP, vm 
DALRADIAN SCHISTS 
123 
second or Blair- Athol Limestone lies next to this quartzite. If the lime- 
stones are identical with those of Donegal, Mayo and Galway, the quartzites 
may doubtless he also regarded as continued in those of the same Irish 
counties, where they form some of the most conspicuous features in the 
scenery, since they rise into such conspicuous mountains as Erigal, Slieve 
League, Nephin, and the twelve Bins of Connemara. 
The age of this vast system of altered rocks has still to he determined. 
It is possilile that they may include some parts of the Torridonian series, or 
even hero and there a wedge of the Lewisian gneiss driven into positi(jn hy 
gigantic disiuptions, like those of the Horth-West Highlands. But there 
can he no doubt tliat tlie schists, quartzites and limestones form an assem- 
blage of metamorphosed sedimentary strata which differs much in variety 
of petrographical character, as well as in thickness, from the Torridonian 
sandstone, and which has not Ijeen identified as the equivalent of any known 
Palasozoic system or group of formations in Britain. It may conceivably 
embrace the Cambrian series of the North -West Highlands, and also the 
sedimentary deposits that succeeded the Durness Limestone, of which no 
recognizable vestige remains in Sutherland or Boss. 
That the metamorphic rocks east of the line of the Great Glen are at 
least older than the Arenig formation of the Lower Silurian system may be 
inferred from an interesting discovery recently made by the officers of tlie 
Geological Survey. A narrow strip of rocks has been found which, from 
their remarkable petrographical characters, their order of sequence and their 
scanty fossil contents (Badiolaria), are with some confidence identified with 
a peculiar assemblage of rocks on the Arenig horizon of the Silurian system 
in the Southern Uplands of Scotland, to which fuller I'eference will be made 
in Chapter xii. This strip or wedge of probably Lower Silurian strata 
intervenes l)etween the Highland schists and the Old Bed Sandstone in 
Kincardinesliire, Forfarshire and Dumbartonshire. It has been recognized 
also, occupying a similar position, in Tyrone in Ireland. Tlie sehists in some 
places retain their foliated character up to the abrupt line of junction with 
the presumably Lower Silurian strata, while in other districts, as at Aberfoyle, 
they have been so little affected that it is hardly possible to draw a line 
between the Highland rocks and those of this border-zone, which indeed are 
there perhaps more metamorphosed than the Highland grits to the north 
of them. The metamorphism of the schists may have been mainly effected 
before the final disturbances that wedged in this strip of Silurian strata 
along the Highland border, though some amount of crushing and schist- 
making seems to have accompanied these disturbances. No trace of any 
similar strip of Palteozoic rocks has ever been detected among the folds of 
the schists further into the Highlands. But some of the Highland roeks 
in the region of Loch Awe lose their metamorphosed character, and pass 
into sedimentary strata which, so far as petrographical cliaractei’s are con- 
cerned, might well be I'alieozoic. 
Until some clue is fo\md to the age of the Younger or Eastern schists, 
quartzites and limestones of the Highlands, it is desirable to liave some short 
