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 anarch. 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 Suarre 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 Cuartes Darwin,* reviewing his observations on cleavage in South Ame- 
rica, says,—‘‘ The cleavage laminee 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 laminz 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 lamine 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 Sorpy, 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 
