838 REPORT—1885. 
place in a more symmetrical manner than in the last-named, so that the 
resulting ‘ cleavage-planes’ are perpendicular to the general direction of 
stratification. The interval between them is from } to }-inch (fig. 11), 
but there is also some tendency in the intermediate parts of the rock to 
split in the same direction. This is explained by a microscopic examina- 
tion, which reveals that, besides the contortions mentioned, there is also 
a system of much smaller contortions, which give rise in like manner to 
a set of less pronounced cleavage-planes at distances of from ;}, to +4,- 
inch apart (fig. 12). Ifthe rock be split along the bedding, which is 
easily done, these microscopic con- 
tortions are seen as fine strize marked 
on the surface of the larger flutings, 
and having the same direction. The 
larger contortions in this rock, and 
the resulting ‘ cleavage ’ may be com- 
pared with an example figured by 
Professor Heim.! 
When a rock-mass becomes con- 
torted by the action of pressure, it 
usually yields in such a manner that 
the contortions into which the planes 
of stratification, or any pre-existing 
planes, are thrown, are of an undu- 
lating form. It seems, however, that, 
under certain circumstances, zigzag, 
instead of undulating, contortions are 
produced, the crests and troughs 
of the contortions being not curved but more or less sharply angled, 
as in fig. 13. Here a shearing motion has taken place in the direction 
parallel to AA’ and BB’, but such shearing has occurred only in certain 
parts of the mass, such as those between AA’ and BB’, or between CC’ 
and DD’, while the other parts, such as those between BB’ and CO’, 
have not been affected. In the kind of contortions considered,” each zig- 
zag is usually unsymmetrical, having a long and a short limb. Excellent 
Fie. 12. 
* Op. cit., Atlas, Taf. xv., fig. 11. 
? Cf. Stapf, Neu. Jahrb. fiir Min., 5c., 1882, Bd. i. 8. 75. 
