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Pre-Cambrian Rocks of Ross-shire. 163 
and this seems now fully borne out by microscopical examination. 
The diorite undoubtedly alters the rocks in contact with it, and 
must therefore have been intruded after the Cambrian and Silurian 
sediments had been deposited. Contact alteration is less evident, if 
at all present, near the granitic mass; and it is not improbable that 
this was intruded like most of the granite veins into the Pre-Cam- 
brian rocks before any of the newer deposits were formed. I am the 
more inclined to think this since a similar rock is found along the 
west coast near Gaerloch, intruded only into the Pre-Cambrian rocks, 
and at Loch Torridon fragments are found in the Cambrian conglo- 
merates, which to all appearances might have been derived from such 
a mass as we have here. That it does not properly, however, belong 
to the true Pre-Cambrian gneiss series,! is evident from the direc- 
tion in which it runs, here, and wherever examined further north 
directly across the strike of the old rocks. The normal strike 
remaining undisturbed in the gneiss rocks, which extend along 
and frequently in contact with both borders of the mass. For example, 
the strike in the Pre-Cambrian rocks is perfectly clear even after 
passing the Hassac Valley; and it is found equally well marked 
directly beyond the Laggan River, and in the undulating ground 
between that and the Glen Docherty heights. Now no series of 
faults would be likely to produce changes, so identical in character 
as these are found to be, continuously along this line: nor indeed does 
it seem at all probable that the strike would change so suddenly, 
be retained afterwards over so exceedingly narrow an area only, and 
then return equally suddenly to the normal direction. That the 
normal strike occurs everywhere in the old floor, where exposed on 
both sides of the mass, is undoubted, and the conditions observed 
at the entrance to the glen are seen equally clearly at the upper 
end—several miles from this point—the granitic mass intervening 
there as at the entrance to sever the connexion between the floor 
on either side. 
Looking at all the evidence, in the three sections, which I made 
across Glen Laggan, I am inclined to think that this granite mass 
belongs to a Pre-Cambrian period, but that it is withal of intrusive 
origin. If this is the case an easy explanation may be given of the 
changes noticed in the section opposite Kenloch Ewe. The quartzites 
lie nearly horizontally where they first touch the granitic mass. A 
sudden bend then takes place as if they were dropped by a fault, and 
a nearly vertical face of the granitic mass is presented towards the 
quartzites. This would, therefore, indicate a faulted junction rather 
than an intrusion into the newer rocks. Another fault is met with 
in a line with the river, and beyond this the old floor is again 
reached. Here it maintains the relationship with the newer rocks, 
which was noticed before the broken ground opposite Kenloch Ewe 
1 Prof. Bonney, in some notes which he has just published (Quart. Journ. Geol. 
Soe. vol. xxvi. p. 95) says, “that all the so-called syenite (except some intrusive 
dykes) is simply a rather granitoid variety of the Hebridean gneiss.” In this he 
includes the granitic rock just mentioned, but the evidence which I have brought 
forward seems to show that this is almost impossible, even if the petrological evidence 
strongly favoured (which it really does not) such a view. 
