

Parr I, Sec, iv.§3.] DEFORMATION. 313 
“epsomites,” or “stylolites.” These are cylindrical or columnar bodies 
varying in length up to more than four inches, and in diameter to 
two or more inches. ‘The sides are longitudinally striated or grooved. 
Each column, usually with a conical or rounded cap of clay, beneath 
which ashell or other organism may frequently be detected, is placed 
at right angles to the bedding of the limestones, or calcareous shales 
= a SS 
=> — ee SS = 
eine et SS ( 
Se 
Se 
—~ == 
" SS : 
SSS oe 
\\ . 5 
\ WAs a ff 
AES ANS 







Fic. 80.—Strophomena expansa, ALTERED BY THE DEFORMING INFLUENCE OF 
CLEAVAGE —LOWER SILURIAN, Cwm IDWAL, CAERNARVONSHIRE. (B.) 
through which it passes, and consists of the same material. This 
structure has been referred by Professor Marsh to the difference 
between the resistance offered by the column under the shell, and 
by the surrounding matrix to superincumbent pressure. The striated 
surface in this view is a case of “slickensides.” The same observer 
has suggested that the more complex structure known as “Cone in 
cone” may be due to the action of pressure upon concretions in the 
course of formation." 
The ingenious experiments of M. Tresca? on the flow of solids 
have thrown considerable light upon these internal deformations of 
rock-masses. He has proved that, even at ordinary atmospheric 
temperatures, solid resisting bodies like lead, cast-iron, and ice, may 
be so compressed as to undergo an internal motion of their parts, 
closely analogous to that of fluids. Thus, a solid jet of lead has 
been produced by placing a piece of the metal in a cavity between 
the jaws of a powerful compressing machine. Iron, in like manner, 
has been forced to flow in the solid state into cavities and take their 
shape. On cutting sections of the metals so compressed, their 
particles or crystals are found to have ranged themselves in lines of 
flow which follow the contour of the space into which they have been 
squeezed. Such experiments are of considerable geological interest. 
They suggest that in certain circumstances, under great pressure, 
the unequally mixed particles of rocks within the earth’s crust may 
1 Proc. American Assoc. Science, 1867. 
2 Comptes Rendus, 1864, p. 754; 1867, p. 809. Mém. Sav. Etrangers, xviii. p. 733 
xx. p. 75. Inst. Mech. Engineers, June, 1867; June, 1878. 
