122 
POPULAR SCIENCE REVIEW. 
and most probably intimately connected with the intrusion of 
the eruptive rocks of the district. 
The causes which are concerned in the production of cleavage 
structure were for a long period involved in obscurity. Some 
of the early observers attributed these phenomena to concre- 
tionary, crystalline, or chemical action, and others to the effect 
of magnetism or electricity ; as, however, the explanations given 
were both vague and unsatisfactory, besides not always consis- 
tent with the supposition that the expounders of these hypo- 
theses were themselves altogether at home in the study of those 
forces which they thus invoked to their aid, it is to be suspected 
that the hypotheses advanced were but another mode of account- 
ing for ignorance. 
It was quite evident, however, that crystallisation could not 
be the cause, since, in all cases of perfect cleavage, no trace of 
crystallisation can be detected even with the microscope, and 
in cases where crystalline structure is developed, the cleavage 
is invariably much more imperfect, or altogether absent. 
The late Mr. Daniel Sharpe was, however, the first who 
pointed out the true road to the solution of this problem, when 
he announced, as the result of his observations, that cleaved 
rocks had undergone a compression of their mass in a direction 
perpendicular to the cleavage planes, and showed that in cleaved 
rocks the particles were always arranged with their flat sides 
parallel to the cleavage planes, and that they were usually 
longer on the line of dip of the cleavage than on the strike. He 
also confirmed what had previously been noticed by Professor 
Phillips, that the fossils, when present in cleaved rocks, were 
always more or less distorted, the distortion being greatest 
where the angle between the cleavage and stratification is 
least. 
The microscopic examination of sections of cleaved and 
uncleaved slate rock showed their structure to be very dis- 
similar. In the latter, as seen in fig. 8 a, PI. LVII., the grains 
of sand, clay, mica, &c., which compose the mass of the rock, 
are seen as if disposed at random, without any trace of defi- 
nite arrangement, whilst fig. 8 6, which shows a section of 
cleaved slate (roofing slate), proves the particles to be distinctly 
arranged and elongated along the line of cleavage of the 
slate. 
Sorby also showed, as figured in fig. 10 a and 6, PI. LVII., that 
analogous differences in structure occur in the cleaved and 
uncleaved devonian limestones from Devonshire, the latter being 1 
a section of an encnnite joint, in which the cells are arranged 
without any order, and retain their normal or equiaxial shapes. 
In the former, however, which is a highly cleaved limestone 
from Kingskerswell, the structure is seen to be entirely changed, 
