454 MR. D. SHARPE ON THE ARRANGEMENT OF THE FOLIATION AND 
northerly line of vertical foliation belonging to the great system of north-easterlv 
strike which occupies so large a part of Scotland ; for to the north-west of that band 
there is only half an arch broken off nearly along its central axis, at a line which mav 
be drawn about N. 25 ° E. from the head of Loch Maree to Loch Eribol in Suther- 
land, and which divides the gneiss of Scotland into two districts of very unequal 
size, distinguished by different directions of their foliation. The gneiss immediately 
to the eastward of the last-mentioned line dips about E. 15° S. at angles rarely 
exceeding 15° or 20°: this dip gradually increases as we proceed eastward, till it 
reaches the perpendicular on the line just traced through the middle of Ross-shire, 
thus forming half an arch only twenty miles wide at its southern, and nearly thirty 
miles wide at its northern extremity, of which the western half is entirely wanting. 
But on the western side of the line between Loch Eribol and Loch Maree-head all 
the gneiss of the main land and that of the island of Lewis strikes towards the north- 
west ; thus the axes of elevation of tlie two districts run nearly at right angles to 
one another. Mr. Cunningham, who has drawn attention to the line commencing at 
Loch Eribol as far as it runs through Sutherland, regarded the gneiss to the east of 
that line as of a more recent formation than that to the westward of it, stating that 
the eastern gneiss often overlies the stratified quartz rock and limestone, which 
never occurs to the westward of the line in question*. 
Although the superposition of the gneiss to the quartz rock and red sandstone in 
other parts of the same line has been mentioned by Macculloch'I', we may require 
further evidence before accepting so remarkable a statement ; there are so many 
districts in the Highlands where secondary and truly stratified quartz rock has been 
confounded with gneiss that a mistake of the kind need create no surprise. It is 
however worth remarking, that the alteration of the Old Red Sandstone into quartz 
rock has taken place principally along or to the east of the line which separates the 
two districts of gneiss, from Loch Eribol to Loch Maree-head and Loch Canon ; 
while the Old Red Sandstone resting on the gneiss to the west of that line is 
unaltered. 
The want of parallelism between the lines of perpendicular foliation and cleavage, 
forming the boundaries of the great arches of gneiss, schist and slate which traverse 
Seotland, is an unexpected phenomenon deserving particular attention. The most 
southerly of these arches, that including Loch Tay, which consists principally of 
mica schist and clay slate, widens slightly in its course westward ; but all the other 
arches, traversing districts consisting for the most part of gneiss, widen considerably 
as they proceed eastward with uniform regularity. The line from Loch Eribol to 
Loch Maree, along the supposed axis of an arch, runs about . . . N. 25° E. 
The perpendicular through Ross-shire, runs about N. 30° E. 
That through Coryaraick, runs about N. 35° E. 
That through Corbuie and the Monaghlea Mountains, runs about . N. 40° E. 
And that through Ben Lawers, runs about N. 50° E. 
* Geognostical Account of Sutherland, p. 96 to 100. f Western Isles, vol. ii. p. 94, and Plate 31, fig. 2. 
