456 
MR. D. SHARPE ON THE ARRANGEMENT OF THE FOLIATION AND 
gneiss of the Isle of Lewis as far as I examined it : there is a vertical plane crossing 
the island N. 45° W. from Broad Bay near Stornaway to Barvas, on the south side 
of which is a broad arch, which I only followed partially ; its central axis is seen at 
Callernish striking N. 20° W,, so that its other boundary may be looked for towards 
the southern end of Lewis. 
Let us now quit these dry details, and turn to the connection between the direction 
of the foliation of the rocks and the physical features of the country, which will 
bring out many points of great interest. The most rugged and elevated hills are 
usually found either along the lines of vertical foliation or where the dip nearly 
approaches the perpendicular. Thus the southern perpendicular plane crosses Ben 
Voirlich, and runs close to Ben Ledi and Ben Venue : the next perpendicular 
described runs through Ben Lawers ; the next through the Monaghlea Mountains ; 
another through Coryaraick, and the northern line through Ben Wyvis: and besides 
crossing these, which are among the highest mountains in Scotland formed of gneiss 
and mica schist, the perpendiculars run along many high ridges, which will be seen 
most readily by reference to the Map. 
The central axis of each of the great arehes described, is also frequently occupied 
by hills of considerable elevation, but less rugged than those on the boundary lines, 
of which the hills on the south side of Loch Tay and the Grampians are the most 
striking examples. 
The lowest ground frequently occurs about half-way between the perpendicular 
walls of the arches and their central axes, and all the great valleys which run 
parallel to the strike of the foliation occupy this position; as instances may be 
quoted the line of Loch Tay, Loch Dochart, and the upper reach of Loch Tyne; 
Strath Spey, from Laggan downwards ; the valley of the Findhorn, and the Great 
Glen ; all of which occupy lines intermediate between the centre and the boundary 
of their respective areas. These great valleys are not cracks analogous to the so- 
called valleys of elevation of the stratified districts ; they seem to be lines of less 
elevation left at the original upheaval of the areas, and due to some unequal opera- 
tion of the upheaving force. The annexed diagram shows a frequent form of outline 
of the country included in each arch of the foliation ; but there are also several cases 
where there is a valley on only one side of the central axis without any correspond- 
ing depression on the other side. 
Perhaps the most striking evidence of the connection of the physical geography 
with the foliation of the rocks, will be found in observing on the Map how many of 
