584 
noticeable that even in the days when what Professor Ramsay 
has called the " Jack-in-the-box " hypothesis was rampant, 
Conybeare and Buckland regarded the anticlinals of the 
Mendip Carboniferous rocks as irrefragable proofs of disturb- 
ance, produced, not by vertical eruptive thrust, but by lateral 
pressure ; and Phillips remarked that " the general absence 
of igneous rocks on the line of the Pennine and Craven 
Faults, at all points (except Ingleton) south of the Cross 
Fell region, is a remarkable circumstance, which ought not 
to be forgotten in reasoning on the agencies concerned in 
producing such disturbances of the strata."* 
The violent contortions of the Lothersdale, Thornton, 
and Skipton limestone, seem at first sight to suggest a 
sudden protrusion, but the fact that so unelastic a substance 
as mountain limestone has been bent at sharp angles without 
breaking, really indicates a gradual and prolonged action. 
I can illustrate this by quoting the results of some small 
experiments which I have lately made. Two pieces of board 
were fitted together in such a manner that they could be set 
at any angle from 0° to 180°, just as you might open a book, 
keeping the letter-press always downwards. Means were 
found of estimating accurately the angle made by the two 
surfaces, and upon this ridge various thin plates of stone 
were placed. Sudden pressure being applied, the angle 
which was sufficient to produce fracture was noted. Then, 
by means of weights, a slow and equable force was tried. 
The following results are noticeable. A thin plate of 
limestone broke at 2^° when pressure was applied suddenly ; 
but at 4°, when the pressure was distributed over an hour's 
time. For a lamina of Bradford Flagstone, the respective 
angles were 3° and 7J°. For a piece of Welsh Slate, 1J° 
and 4°. For a piece of Skiddaw Slate, 3£ 0 and 5°. For a 
layer of shale, found in the roof of the Halifax hard bed, 
* Geology of Yorkshire, vol. ii., p. 253. 
