136 Reviews—A. Daubrée’s Experimental Geology. 
held. But the cause of joints is still a dispute, wherein widely 
different views are maintained. Crystallization, or processes analo- 
gous to it, contraction, and mechanical strains, each have their 
various advocates. M. Daubrée marshals the whole array of his 
arguments and experiments on the side of mechanical actions. He 
adduces the generally uniform direction of joints over wide areas, 
and through varying rocks; the occasional distortion of the fossils 
in their neighbourhood (on the authority of Professor Harkness) ; 
and the well-known and significant fact that in many conglomerates 
they cut clean through pebbles and matrix alike, a fact difficult to 
explain on any hypothesis of contraction. Also he contends that 
faults and joints are connected by so many common characters, and 
graduate so perfectly into each other, that a common origin must 
be sought for them. The character in both on which most stress is 
laid, and towards which the experiments are most generally 
directed, is their arrangement in extensive parallel systems. 
Experiments imitating undulations gave no satisfactory result, but 
torsion and compression reproduced most of the peculiarities to be 
accounted for. For torsion, plates of glass were coated with thin 
paper and submitted to a wrench. Figures are given of the systems 
of fractures produced, which do in many respects recall the arrange- 
ment of systems of joints. Besides the visible fractures, systems 
of concealed planes of separation or weakness arise, which may be 
brought to ight by the impulse of a shock, and answer to concealed 
joints or cleavage. 
For pressure, blocks of a mixture of plaster and wax were longi- 
tudinally compressed by a powerful force. When the blocks had 
the proper degree of rigidity, after bulging a little, they yielded 
to the stress by the production of oblique fractures, and the con- 
sequent slipping gave rise to other fractures at right angles to 
the former. The ultimate results, shown in beautiful plates, and 
described at length, are some larger fissures making angles of 45° with 
the direction of pressure, systems of smaller crevices parallel to one 
or other of these, and yet other systems similarly aligned of cracks 
closed, invisible, and almost infinitely close and numerous. ‘These 
are compared to larger and smaller faults, and joints and cleavage. 
The obvious objection to the pressure-theory of faults is the old 
observation that they generally ‘hade to the down-throw.’ M. 
Daubrée answers that the exceptions are neither few nor un- 
important ; and that a horizontal shift can occasionally be proved 
by the directions of the slickensides. Probably he considers little 
disturbed regions as homologous to the bulges on his blocks, where 
the bulge must create ruptures by tension. He seems also to com- 
pare the fractures of such regions with the results of his experiments 
on torsion. In truth any irregular settlement of beds can scarcely 
fail to produce actions analogous to those of a wrench. 
For pressure as a cause of joints there seems much to be said. 
If it be objected that joints ought in that case to show slickensides, 
M. Daubrée gives instances where they do. Besides, the motion 
which relieved the stress might well be so minute as to cause no 
