RESEARCHES ON THE CONTORTION OF ROCKS. 
29 
the case appeared from the raising of the free edges of the 
slab above the surrounding pavement, which was elsewhere 
fairly level. But all doubt on this point was removed by 
subsequent measurement. A year and five months later the 
centre of the slab had sunk T 7 g- of an inch more, relatively to 
the ends. 
Again, a friend communicated to me the experience of 
slaters, who find that when by yielding of the timbers the 
surface of an old roof has become bowed, the tiles are distorted 
to such an extent that they will not lie flat upon a new roof. 
Old stone tiles (I do not know whether the same is true of 
slates and brick tiles) are often rendered perfectly useless in 
this way, however sound they may be. It is perhaps unneces- 
sary to cite other similar cases. Every observant architect 
and engineer can give from his own experience facts of in- 
terest in reference to the influence of long-continued pressure 
upon an unsupported edge of stone. 
The magnificent instances of contortion sometimes displayed 
in coast sections are certainly more impressive, but perhaps 
less wonderful in reality, than the cases on record of distorted 
pebbles. The unlimited effects of long pressure are nowhere 
so clearly demonstrated as in the bending of round or oval 
masses of small size. Instances of pebbles elongated in the 
direction of planes of cleavage occur at Llyn Padarn, near 
Llanberis, in the Lake District and elsewhere. But the most 
remarkable cases of alteration of figure effected by pressure are 
those described by Dr. Hitchcock, Mr. G-. L. Yose, and others, as 
occurring in New England. In Vermont, Maine, Massachusetts 
and Rhode Island, are found conglomerates where sometimes 
for hundreds of square feet every pebble, whether of granite, 
sandstone, schist or quartz, has been flattened. Occasionally 
one pebble has been driven into another, so as to indent it or 
squeeze it into a semicircular form, yet without fracture. Some 
of the examples figured resemble soft cakes jammed together 
into one mass with unyielding stones, so freely do they curve 
round in layers and adapt their shape to the various lines of 
force. Yet plasticity in any ordinary sense of the word is out 
of the question. These very pebbles are water-worn and some 
of them cleaved. Not a few are rolled fragments of plutonic 
rock. (See Plate LXXX. Figs. 2, 3.) 
The connection between contorting and cleaving force is not 
quite clear. It would seem that when a certain freedom of 
extension is allowed even the most intractable substances yield 
and change their form. But if the compressed mass be 
wedged up so tightly that change of figure is impossible, the 
individual particles seem to revolve upon their axes, and 
arrange themselves, as coins would do, with their principal 
