448 
MR. D. SHARPE ON THE ARRANGEJMENT OF THE FOLIATION AND 
volutions, the foliation will usually be found to be arranged in great arches, in which 
the inclination of the folia increases as they are more distant from the centre until 
they reach the perpendicular ; the whole thus forming parts of a flat arch bounded 
by two perpendiculars, a theoretical restitution of which is shown in figure 2. 
Beyond the perpendiculars which form the boundaries of the arch there are usually 
comparatively narrow bands, in which the dip of the folia is irregular ; these are suc- 
ceeded by other perpendicular planes, usually parallel to the first, which form the 
boundaries of other arches standing on each side of the first, but which are often 
incomplete. As the planes and curved surfaces are extended in the direction of the 
strike, each arch here represented is the section of a semi-cylinder, which is often of 
great length. 
The perpendicular planes of foliation frequently run along important ridges of 
hills, which has brought them into notice ; while the lower and flatter central por- 
tions, in which the general arrangement is disguised by the contortions, have attracted 
less attention. Hence several geologists have mentioned the fan-like or radiating 
structure in the metamorphic schists of the Alps, in which the folia in the central 
crests are vertical, and on the two flanks inclined inwards^;” but it has been over- 
looked, that the fan-like form is produced by combining portions of two adjoining 
arches : an inspection of the sections, fig. 1 to 5, will convince every one that the 
arch is the figure of real importance in the arrangement, and the fan-shaped form 
only catches the eye when seen without the rest of the figure. 
Analogy between Foliation and Cleavage. 
If the arrangement of the foliation which has been just described be compared 
with that of the cleavage planes of the true slates which I have explained elsewhere'|-, 
the two phenomena will be found to correspond in all their main features ; both 
forming great arches bounded by vertical planes, with the peculiarity that in both 
the dip of the divisional surfaces is least at the central axis, and gradually increases 
towards the boundaries till it becomes perpendicular. The difference between the 
two lies in the convolution of the layers of gneiss and schist, which contrasts strongly 
with the regularity of the flat parallel surfaces produced in slate by the cleavage; 
yet the difference is rather of degree than of principle ; for in Devonshire, where the 
cleavage forms an unbroken arch with a diameter of sixty miles, and where we can 
judge of its real characters better than in more disturbed districts, the central part 
undulates in low curves:}:, which are evidently analogous to the convolutions of the 
central portions of the arches of foliation. 
Mr. Darwin has stated, that when in various parts of South America he met with 
laminated and foliated rocks either alternating with or near to one another, the 
cleavage and foliation were generally parallel ; and from this circumstance, and an 
* Dakwin, South America, p. 164, where Von Buch and Studer are quoted. 
j- Journal of the Geological Society, vol. hi. p. 92. J Journal of the Geological Society, vol. ih. p. 95. 
