RESEARCHES ON THE CONTORTION OF ROCKS. 
27 
“ flexible limestones ” are very difficult to bend permanently. 
One specimen exhibited for many years in a public museum, 
with the two ends supported and the centre slightly depressed 
to show its flexibility, does not present any visible deflection 
when placed on its edge. 
Thin natural laminae of flagstone from the coal measures 
were also tried. Various specimens were selected according to 
their texture and mineralogical character, but none yielded 
important results. It will be seen further on, from other evi- 
dence, that considerable deflection has been unintentionally pro- 
duced in these flagstones, but as yet I have never succeeded in 
bending thin plates more than 8'. Slates of various kinds 
have also proved very intractable, an interesting and not unex- 
pected result. No material has yet done so well in my hands 
as carefully cut slabs of mountain limestone 4 in. x 3 in. and 
*07 in thickness. 
The frequent destruction by spontaneous fracture of bent 
plates when removed from the machine seems to imply that an 
indefinitely protracted and uniformly contorting force is needed 
to produce unbroken curvature, such as that on the coast of 
Berwickshire. The experiments next to be related tend to 
show that resistance on all sides diminishes the risk of fracture. 
While designed to answer other purposes, the precautions de- 
scribed and the result attained serve to strengthen the opinion 
that unbroken anticlinals and synclinals are only formed under 
a considerable weight of superjacent strata. 
Anxious to imitate the natural condition of lateral pressure 
more closely, and at the same time to preserve well-contorted 
specimens for reference, I tried another method, which ulti- 
mately yielded interesting results. My object was to apply 
pressure to the edges of a slab of stone and overcome the ten- 
dency to fracture by embedding it in a matrix of some tena- 
cious substance. The precaution was especially necessary in 
this second series of experiments. It is easy to see that when 
deflection begins the bending force is increased in a high ratio. 
We have the pressure acting upon the slab, not as a force 
transmitted through its plane, but concentrated upon the 
middle point, the two halves acting as levers. As the contor- 
tion proceeds the strain increases rapidly, and in practice it is 
found that no graduated pressure can be contrived sufficiently 
delicate to avoid sudden fracture. 
To overcome such difficulties as these I imbedded thin slabs 
of limestone in pitch and fitted them into a cast-iron box, the 
two sides of which were removed. One end was cut to allow a 
screw to travel through it, and within was a plate of iron which 
could be moved along by the pressure of the screw so as to 
tighten the slab within the box. By this means pressure was 
