CLEAVAGE OF THE ROCKS OF THE NORTH OF SCOTLAND. 
447 
Contortions of the Foliation. 
When an observer enters any district of gneiss or schist in search of order in the 
arrangement of their folia, liis first impression will be that of despair; so numerous 
are the convolutions of both rocks, varying in importance from curves which have 
determined the outline of mountain ridges to folds no larger than those in the 
drapery of a curtain, and so complicated and involved do they appear, that it seems 
hopeless to seek for any general plan among them. These contortions have often 
been figured*, but no representation has ever shown a tithe of their complication; 
they are the most numerous along the dip of the foliation, but they often occur on 
the line of strike also ; and in spots where the rock is only exposed to a small extent, 
it is often impossible to judge of the prevalent direction of the foliation. 
But where large sections are well seen, a more extended observation will show that 
even where the gneiss is most convoluted, there is still a general direction of the 
folia or layers of which it is composed, which may be caught by disregarding the 
minor folds, and fixing the attention only on the larger curves formed by a combina- 
tion of many of the smaller flexures. This will sometimes be more readily seen from 
a distance, at which the minuter complications are lost sight of. The simpler lines 
of curve thus obtained may perhaps still be in themselves complicated; but by 
repeating the process of simplification and disregarding the secondary flexures, a 
series of lines of dip are at last obtained, sufficiently simple to admit of classification 
and of representation in a reduced drawing. 
The observations of the direction of the strike must be corrected by repetition ; 
and where the lines of strike are most curved, the gneiss will often be found rising 
into elliptical bosses, the longer axes of which are all parallel and give the general 
direction of the strike of the district. 
Gneiss and mica schists are not equally contorted throughout; the convolutions 
are usually most complicated where the inclination of the folia is slightest ; where 
their dip is steeper the rock is usually less contorted, and where the foliation is per- 
pendicular it usually follows true planes without any distortion either on the dip or 
strike. The granitic varieties of gneiss, in which the foliation is slightly marked, are, 
I believe, more common where the angle of inclination is slight ; and the separation 
of the minerals is usually well marked where the gneiss is either perpendicular or 
highly inclined. Thus observations taken where the foliation is perpendicular, admit 
of much greater accuracy than can be obtained where it is slightly inclined. 
Arrangement of the Foliation in Arches. 
If any extensive region of gneiss or schist is crossed in a direction transverse to 
the strike, and the larger and bolder curves noted to the exclusion of the minor con- 
* Macculloch, Western Isles, Plate 12, and frontispiece; also Geological Transactions, vol. ii. Plate 31 
and 32. 
3 M 
MDCCCLII. 
