ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 85lL 
Wales, Cornwall, and Devon, and the Cumbrian Mountains,! and subse- 
quently in the Scottish Highlands. He maintained that in each of the 
districts, which he regarded as distinct areas, there are certain lines of 
strike, many miles apart, along which the cleavage is vertical; that on 
each side of such a line the cleavage dips towards it, and at continually 
lower angles, until midway between two such lines is a zone of horizontal 
cleavage ; so that over such an intermediate place the cleavage-surfaces, 
if carried on continuously, would pass in broad flat arches. With such 
an arrangement, low angles of cleavage-dip would be much more preva- 
lent than they are found to be. Professor Phillips criticised Mr. Sharpe’s 
theory and some of the sections on which it was based, and even these 
sections themselves only bear out the author’s view in a very limited 
degree. It is curious, too, that M. Laugel* employed Mr. Sharpe’s data 
to verify his own quite different theory of a sheaf-like or fan-like arrange- 
ment of the cleavage-planes of a district, which is much more in agree- 
-ment with recorded observations. According to Mr. Sharpe‘ this 
appearance is due to the junction of two of his arches, but certainly the 
prevailing high dips in most districts where cleavage is well developed 
point to the fan-like arrangement as the essential part of the phenomena. 
This latter peculiarity is specially characteristic of mountain systems, 
where, as noted by Dr. Darwin,* the cleavage-planes on the flanks of the 
mountains ‘ frequently dip at a high angle inwards.’ In a symmetrical 
mountain complex also there is usually a parallelism between the cleavage- 
planes and the axial planes of the folds into which the strata are thrown, 
as remarked by Professor H. D. Rogers.® In less disturbed districts the 
fan arrangement is less perfect, and any connection between the direction 
of the cleavage-planes and the position of the folds of the rocks is, as a 
rule, not to be made out except occasionally on asmall scale.7 A traverse 
through the slate-districts of North Wales seems to show that the 
cleavage-planes, as it were, oscillate from one side of the vertical to the 
other when followed in a direction perpendicular to their strike, as if 
there were a series of imperfect fans; some of the Geological Survey’s 
sections show the same character. 
Mr. Sharpe’s presumed arch-arrangement was used by him to support 
an elevation theory of cleavage, which was briefly that a mass of fluid 
igneous matter, forced upward through a fissure coinciding in direction 
with the axis of the ‘area of elevation,’ had compressed the surrounding 
rocks; the pressure being supposed to act radially, the resulting cleavage- 
planes, which are at right angles to the direction of pressure, would form 
a flat arch. The fan-arrangement, on the other hand, seems to connect 
itself in a simple manner with the lateral compression theory; ® for a 
mountain mass thus elevated would be acted on by lateral thrust some- 
what like that in an arch of brickwork; the cleavage-planes, being per- 
pendicular to the thrust at each point, would be arranged like the planes 
which separate successive bricks in the arch, i.e. in a radiating or fan-like 
manner. 
1 Quart. Journ. Geol. Soc., vol. v. p. 112 (1849). 
* «On the Arrangement of the Foliation and Cleavage of the Rocks of the North 
of Scotland,’ Phil. Trans., 1852, p. 445. 
* ‘Du clivage des roches,’ Bull. Soc. Géol. Fran., sér. 2, t. xii. p. 363 (1855). 
* Phil. Trans., 1852, pp. 447, 448, &c. + 
° Geol. Obs. in South America. p. 164 (1846). 
‘§ Trans. Roy. Soc. Edinb., vol. xxi. p- 447 (1856). 
7 See, ¢.g., Dr. Sorby’s Ilfracombe section, loc. cit. 
* Cf. Pilar, Grundziige der Abyssodynamik, 1881. 
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