144 
BEYIEWS. 
In tlie section on Cleavage tlie meclianical theory is advocated, and 
many important facts connected with the subject stated:—• 
“ One of tlie most striking effects of cleavage is the distortion it produces on fossils 
or other small bodies embedded in the rocks, lengthening and pulling them, as it were, 
in the direction of the cleavage, and contracting them in the opposite direction. Relying 
on these facts, which were first distinctly noticed by Professor John Phillips, Mr. Sharpe 
attributed the production of cleavage to the action of great forces of compression squeeze 
ing the particles of rock in one direction, and lengthening them in the opposite.—‘ Quar¬ 
terly Journal, Geological Society,’ vol. iii. p. 87. Mr. Darwin, also, from his observations 
in South America, formed similar ideas as to the origin of cleavage, and speaks of cleavage 
planes as being probably parts of great curves, of such large radius as that any portions 
of them that can be seen at one view appear to be straight. More recently, Mr. Sorbv 
resting on the fact of beds of sandstone which occur in slate being contorted, and their 
dimensions being contracted at the sides, and expanded at the tops and bottoms of the 
curves, the axes of which curves coincide in direction with the cleavage planes, while 
the beds of slate above the sandstone are little or at all bent, shows that the particles of 
the slates must have been compressed at right angles to the cleavage planes, and length¬ 
ened along them, so as to allow of their being squeezed into the same contracted space as 
the sandstones, without much bending of the surfaces of the beds.—See ‘ New Philosophi¬ 
cal Journal, 1853,’ vol. iv. p. 137 ; or Lyell’s Manual, oth edition, p. 611. 
“ By microscopical examination, Mr. Sorby found that the minute particles of clay- 
slate were either lengthened in the direction of the cleavage planes, or that those minute 
particles which were of unequal dimensions were so re-arranged as that their longer di¬ 
mensions coincided with the planes of the cleavage. 
“ Professor Sedgwick at one time thought that he could perceive a tendency to a 
symmetrical arrangement of the inclination of the planes of cleavage with respect to the 
axes of lines of elevation, the dip of the cleavage being inwards on each side of the moun¬ 
tain ranges. He afterwards, however, saw reason to abandon this conclusion. Mr. 
Darwin speaks of the fan-like arrangements of the- cleavage planes which have been de¬ 
scribed by Yon Buch, Studer, and others; and Mr. Sharpe says that this apparent fan¬ 
like arrangement is due to parts of two contiguous curves meeting where their adjacent 
sides become perpendicular. But we must refer the reader to his paper on this subject, 
in the third and fifth volumes of the Journal of the Geological Society before quoted, 
and in the Philosophical Transactions for 1852. A second cleavage plane cutting across 
the first at right angles, and also across the bedding, is described by Mr. Sharpe in his 
second paper on Cleavage in the Geological Journal, vol. v. p. 3, and was also long before 
observed and mentioned by Professors Sedgwick, Phillips, and others. Mr. Sharpe attri¬ 
butes this likewise to compression. 
“ The subject has recently been investigated by Professor Tyndal, who, in a paper 
in the Philosophical Magazine, vol. xii., distinctly refers the origin of cleavage to the 
same force of compression, acting at right angles to the cleavage planes, that Mr. Sorby 
and Mr. Sharpe had referred it. Professor Haughton, in a paper in the same volume, has 
deduced mathematically a value for the compression of the rocks, from examining the 
amount of distortion suffered by fossils in some particular instances in consequence of 
this compression. 
“ There seems indeed now little doubt that mechanical compression is the true cause 
of cleavage; but the whole subject requires still more accurate and detailed observations 
than have have yet been made on it. I have seen reason to suspect—in some districts of 
North Wales, for instance—that subsequent movements and dislocations have affected 
large cleaved districts in such a way as may have altered both the dip and strike of the 
cleavage from their original position. Direct observation then, now, will only lead us 
astray, unless it be corrected by a more accurate knowledge than we yet possess of the 
amount and direction of these dislocations, and of their relative age compared with that 
of the cleavage. The tip of the cleavage especially is very easily mistaken, unless it be 
observed in very clear and deep excavations. Superficial causes have frequently affected, 
and sometimes completely reversed it to very considerable depths, as may be seen in 
Fig. 58. 
