206 SOLDUES TiO Ke SACD ANTES: 
ticles of the compressed part of the iron bar. But rock beds 
are usually composed of different mineral constituents, which 
differ from one another in strength, in hardness, in brittleness, in 
elasticity, and in size. The necessary rearrangement of the 
mineral particles more largely affects the weak, small particles 
than the large, strong ones. However, as shown by microscopical 
study, where a district is closely folded, no particle of a rock 
stratum, small or great, simple or complex, weak or strong, 
FIG. I. 
escapes the effects of, or fails to take part in, the necessary read- 
justment of folding. 
If a rock stratum could be bent without fracture in such a 
position that the superincumbent weight were slight, about one- 
half of the bed, like the iron bar, would be elongated, and the 
other half would be compressed. Between the two there would 
be a neutral plane. 
As rock beds are brittle they act differently from an iron bar 
when bent to any considerable degree. Beginning at the middle 
of the mass in the trough or crest of the fold and passing toward 
the convex surface, the first lamina is under tension, the second 
under greater tension, and so on, each stratum being stretched 
more than the preceding. The tensile force may go beyond the 
limit of elasticity and radial cracks be formed. (Fig. 1.) Begin- 
ning again at the center and passing toward the concave surface, 
the first layer is under compression, the second under greater com- 
pression, and so on, each stratum being more severely squeezed 
