CLEAVAGE OF THE ROCKS OF THE NORTH OF SCOTLAND. 
449 
occasional tendency in stratified slates to exhibit an incipient mineralogical change 
along the planes of their cleavage, he infers “ that foliation and cleavage are parts 
of the saaie process : in cleavage, there being only an incipient separation of the 
constituent minerals ; in foliation, a much more complete separation and crystal- 
lization*.” 
Along the southern border of the Highlands, a band of slate, in which the stratifi- 
cation and cleavage planes are equally visible, rests on foliated mica schist, affording 
an excellent opportunity of testing these views. The result of an examination of 
that district, fully bears out Mr. Darwin’s opinion : where the two formations come 
together, the cleavage and foliation are not only conformable at the junction, but 
combine to form an arch in common. I have selected four sections to illustrate this 
conformity under different circumstances : at Dunkeld, and on the east side of Loch 
Lomond, fig. 1 and 3, an arch formed principally of the foliation of the mica schist, 
is completed by the addition of the cleavage of part of the slate on its southern 
flank. In Strath Earn and the lower part of Glen Shee, fig. 4 and 5, the foliation of 
the mica schist, besides forming a complete arch, supplies part of the imperfect por- 
tion of another arch to the south, in which it is combined conformably with the 
cleavage planes of the slates : a similar conformity between the foliation and cleavage 
is found throughout ; but the stratification of the slate, indicated in the sections by 
the dotted lines, is unconformable to the foliation of the mica schist, and is of no 
account in the question under consideration-l-. 
An additional proof that one cause has produced both the foliation and cleavage, 
is found in the continuation of the same divisional planes through clay slate and 
mica schist from Loch Katterin to Glen Shee ; these are alternately planes of 
cleavage in the slate and of foliation in the schist ; the boundary line between the 
formations being deflected, while the divisional planes keep a direct course and pass 
from one rock to the other several times without changing their direction or their 
inclination. When Professor Sedgwick announced the persistence of cleavage planes 
through beds of different mineral character, it was at once admitted that the uniform 
cleavage of the various beds must be regarded as one process. There is the same 
reason for connecting together foliation and cleavage, now that the foliation of the 
schist is shown to be continuous with the cleavage of the slates. For these reasons 
the two phenomena will now be described together. 
Geographical Arrangement of the Foliation and Cleavage. 
It is obvious, from what has been stated of the greater distinctness and freedom 
from contortion of the foliation when vertical, that in studying its arrangements 
over any district, it will be best to begin by determining the boundary lines of the 
* Geological Observations on South America, p. 155 and 165. 
t Not to encumber this paper with details foreign to its principal object, I reserve an account of the stratified 
rocks of the Highland border for a communication to the Geological Society. 
3 M 2 
