Hacks of the North of Scotland. 85 



series of simple curves, obtained by observing the general 

 direction, and disregarding the minor and more complicated 

 folds. The convolutions are usually greatest where the dip 

 is slightest, but where the foliation is vertical, or nearly so, 

 it usually follows true planes without contortion ; thus the 

 most correct observations are those taken where the foliation 

 is vertical. 



When the foliation of gneiss and schist is traced over ex- 

 tensive areas, and the minor convolutions disregarded, it is 

 usually found to form arches of great length, and many miles 

 in diameter, bounded by vertical planes, between which the 

 inclination increases with the distance from the axis. Each 

 arch is succeeded by a narrow space, in which the dip is irre- 

 gular, and beyond which another arch commences of a form 

 similar to the first. Portions of two adjoining arches seen 

 without the rest form the fan-like structure observed by seve- 

 ral geologists. The arrangement of the foliation in arches 

 corresponds with that of the cleavage of the true slates pre- 

 viously described by the author, except in the greater convo- 

 lution of the gneiss and schist. 



Along the southern border of the Highlands, a band of 

 stratified clay-slate rests on mica schist ; at the junction, the 

 foliation of the schist conforms to the cleavage of the slate, 

 and the two together form an arch, but there is no connec- 

 tion between the stratification of the slate and the foliation ; 

 moreover, the divisional planes cross from one rock to the 

 other, without change of direction, being planes of foliation 

 in the mica schist, and of cleavage in the slate ; these facts 

 confirm Mr Darwin's opinion, that cleavage and foliation are 

 due to the same cause. 



The author describes the parallel arches of foliation which 

 cross the Highlands, illustrating his description by sections 

 and a map, on which they are laid down, and tracing in 

 detail the vertical planes which bound the arches. Com- 

 mencing on the south, the first vertical plane runs about four 

 miles within the Highland border, with a mean direction of 

 about N. 55° E. : it crosses more than once the junction of 

 the clay slate and mica schist. South of this plane the 

 cleavage of the slate forms the beginning of an arch, which 



