CLEAVAGE OF THE ROCKS OF THE NORTH OF SCOTLAND. 451 
Dimkeld ; through mica schist at the Brig-o~Cally with a strike of N. 50° E., beyond 
which spot I did not follow it. Thus along the Highland border the foliation of the 
mica schist and the cleavage of the slate are both vertical, along lines so closely cor- 
responding, that they may be considered as continuous, which is perhaps the strongest 
evidence that can be adduced to show the identity of the causes which produced the 
two phenomena. 
To the south of the perpendicular which has been traced above, the cleavage hangs 
over to the south, with a steep dip of about N.N.W., thus forming the commence- 
ment of another arch, which is broken olf abruptly along the line of junction of the 
clay slate and Old Red Sandstone, in the manner shown at the southern extremity of 
the sections, fig. 1 to 5. But on the north side, the cleavage, and then the foliation 
dip about S.S.E. towards the perpendicular for a space of five or six miles, first at a 
high angle, and then at a lower inclination accompanied with many waving contor- 
tions ; the dip then changes to about N.N.W. for a similar space of five or six miles, 
and again increases till it reaches the perpendicular on a line nearly parallel to the 
first, the whole forming an arch varying from ten to twelve miles in width, composed 
partly of cleavage and partly of foliation : the central axis of this arch runs along 
the high ridge of hills on the south side of Loch Tay. 
The perpendicular plane which forms the northern boundary of the arch just 
described is broken and often irregular in its direction, and entirely confined to the 
gneiss or mica schist, but as I crossed it at distant intervals I can only map its 
course approximatively. The first point observed on the west was in Strath Fillan, 
about one and a half mile south of Tyndrum, where a vertical foliation runs about 
N. 50° E. tlirough a hard and very quartzose gneiss: the same perpendicular plane 
runs N. 45° E. through Ben Lawers ; N. 60° E. at the ridge between Strath Tay and 
Glen Tummel, a little to the east of Sehehallion ; crosses the pass of Killicrankie in 
a direction very little north of east, and runs N. 35° E. across Glen Shee, about a 
mile below the Spittal ; — a line drawn N. 50° E. from the western point observed in 
Strath Fillan to Killicrankie passes through the intermediate spots named, and gives 
N. 50° E. as the general bearing of this vertical plane of foliation, leaving out of 
aceount the observation in Glen Shee, where the plane seems to have been thrown 
to the south by some local disturbance. 
If this line were continued westward from Tyndrum, it would pass near Inverary, 
but some eruptive masses of porphyry on the west side of Loch Fyne have destroyed 
the regularity of the foliation in that neighbourhood ; the vertical line, however, re- 
appears further west and crosses Knapdale, where it attracted Macculloch’s atten- 
tion : the same author also describes the foliation of the schists meeting in a roof- 
like form down the middle of Cantyre. Thus he has pointed out both the central 
axis of the arch just described, as well as its northern border, without observing the 
arched arrangement of the foliation*. 
* Western Isles, vol. ii. p. 287. 
