OF THE MORE DISTURBED ZONES OF THE EARTH'S CRUST. 459 



the distance from the vertically dipping cleavage increases. This is his expla- 

 nation of the fan-like arrangement of dip noticed in some countries. " This 

 regularly descending series of planes being found on each side of parallel lines of 

 vertical cleavage, the two series either meet in the centre in a sort of anticlinal 

 axis, or coalesce into an arch. The planes between two lines of vertical cleavage 

 appear to form a complete whole, and the area bounded by the vertical cleavage, 

 may be considered as belonging to one system of cleavage, and may be called an 

 area of elevation of the cleavage." He thinks the cleavage planes are really parts 

 of great curves, which, if completed, would represent a series of semicylinders 

 turned over a common axis. 



Mr Shaepe thinks " that there is reason to believe that all slaty rocks have 

 undergone a compression of their mass in a direction perpendicular to the planes 

 of cleavage," connecting with this view his supposition that the cleavage areas are 

 great anticlinal waves. He supposes that the compression of the slaty mass, and 

 its expansion in the direction of the cleavage dip, have been due to the stretch- 

 ing of the strata in the direction of the curve representing the cleavage dips. 



Mr Charles Darwin, * reviewing his observations on cleavage in South Ame- 

 rica, says,—" The cleavage laminae range over wide areas with remarkable uni- 

 formity, being parallel in strike to the main axes of elevation, and generally to 

 the outlines of the coast." He recognizes the fact that the cleavage planes fre- 

 quently dip at a high angle inwards, and he cites an instance of cleavage dip, in 

 the mount at Monte Video, where " hornblendic slate has an east and west 

 vertical cleavage, with the laminae on the north and south sides near the summit 

 dipping inwards, as if the upper part had expanded or bulged outwards." Mr 

 Darwin first proposed the term foliation for the laminae in gneiss and other crys- 

 talline rocks, or the alternating layers or plates of different mineralogical compo- 

 sition. He pointed out the parallelism of the planes of foliation of the mica 

 schists and gneiss with the planes of cleavage of the clay-slate in Tierra del 

 Fuego and Chili, as seen by him in 1835. Darwin conceives that foliation may 

 be the extreme result of the process of which cleavage is the first effect, or that 

 the crystalline form may have been most energetic in the direction of cleavage. 

 He further suggests, " that the planes of cleavage and foliation are intimately 

 connected with the planes of different tension to which the area was long sub- 

 jected, after the main fissures or axes of upheavement had been formed, but be- 

 fore the final cessation of all molecular movement," " and that this difference 

 in the tension might affect the crystalline and concretionary processes." 



Mr Sorby, adopting the mechanical theory of cleavage, maintains that it 

 varies directly as the mechanical changes, and inversely as the chemical (mole- 

 cular) changes, which the strata have undergone. He thinks he has shown that 

 the cleavage of certain limestones, microscopically examined by him, varies di- 



* See Geological Observations on South America. 

 VOL. XXI. PART III. 6 H 



