592 
ELEMENTS OF GEOLOGY. 
hibit the same phenomenon when mixed with water: and 
what occurs in our experiments on a minute scale may occur 
in nature on a great one.”^ 
Mechanical Theory of Cleavage. —Professor Phillips has re¬ 
marked that in some slaty rocks the form of the outline of 
fossil shells and trilobites has been much changed by distor¬ 
tion, which has taken place in a longitudinal, transverse, or 
oblique direction. This change, he adds, seems to be the re¬ 
sult of a “ creeping movement ” of the particles of the rock 
along the planes of cleavage, its direction being always uni¬ 
form over the same tract of country, and its amount in space 
being sometimes measurable, and being as much as a quarter 
or even half an inch. The hard shells are not affected, but 
only those which are thin.f Mr. D. Sharpe, following up the 
same line of inquiry, came to the conclusion that the present 
distorted forms of the shells in certain British slate rocks 
may be accounted for by supposing that the rocks in which 
they are imbedded have undergone compression in a direc¬ 
tion perpendicular to the planes of cleavage, and a corre¬ 
sponding expansion in the direction of the dip of the cleav- 
age-t 
Subsequently (1853) Mr. Sorby demonstrated the great ex¬ 
tent to which this mechanical theory is applicable to the 
slate rocks of I^orth Wales and Devonshire,§ districts where 
the amount of change in dimensions can be tested and meas¬ 
ured by comparing the different effects exerted by lateral 
pressure on alternating beds of finer and coarser materials. 
Thus, for example, in the accompanying figure (Fig. 627) it 
will be seen that the sandy bed cl /* * * § , which has offered great¬ 
er resistance, has been sharply contorted, while the fine¬ 
grained strata, a, 6, c, have remained comparatively unbent. 
The points d and fin the stratum d f must have been origi¬ 
nally four times as far apart as they are now. They have 
been forced so much nearer to each other, partly by bending, 
and partly by becoming elongated in the direction of what 
may be called the longer axes of their contortions, and last¬ 
ly, to a certain small amount, by condensation. The chief 
result has obviously been due to the bending; but, in proof 
of elongation, it will be observed that the thickness of the 
bed df is now about four times greater in those parts lying 
in the main direction of the flexures than in a plane perpen- 
* Letter to the author, dated Cape of Good Hope, Eeb. 20, 1836. 
t Report, Brit., Assoc., Cork, 1843, Sect. p. 60. 
t Quart. Geol. Journ., vol. hi., p. 87,1847. 
§ On the Origin of Slaty Cleavage, by H. C. Sorby, Eclinb. New Phil. 
Journ., 1853, vol. Iv., p. 137. 
