SCATSTOSITY AND SEATY CLEAVAGE 447 
imbedded, or that the flattened particles themselves are cleava- 
ble parallel to their greatest extension. If one supposes the 
material from which slate is made to be a soft mud containing 
minute sand grains, it seems plausible to assume that in indurated 
slate also the matrix is weaker than the enclosed particles. But in 
the analogous dynamo-metamorphosed conglomerates the matrix 
is often firmer than the pebbles and there is no reason to suppose 
than this characteristic is not shared by the fine-grained masses. 
In such cases no cleavage could result from flattening unless the 
larger grains are cleavable in one direction. This could scarcely 
happen unless they were mainly mica scales and I do not think 
that true cleavage would result even in that case. It does not 
appear to me that any closer approach could be made to slaty 
cleavage by flattening the particles, even in a weak matrix, than 
is presented in natural sandstones; for in these also the mica 
scales and thin particles of quartz are in the great majority of 
cases parallel to the bedding. Now it is true that two beds of 
sandstone often split apart from one another with some readiness 
though the fractured surfaces do not resemble those of slate. 
This, however, is not in point. If one tries to split a portion of 
a single, uniform bed of sandstone, it requires careful observa- 
tion to detect greater facility of cleavage in one direction than 
another. Thus the mere fact of parallel orientation does not 
necessarily lead to slaty cleavage. 
Tyndall’s experiment on wax seems well suited to decide 
between the old and new theories. It is also one suitable for 
performance by students.’ If the flattening theory is correct, 
compressed cakes should split entirely across, and at least as well 
at the center as at the edges. If on the contrary I have cor- 
rectly elucidated the problem there should be a small central 
core to each compressed cake unaffected by cleavage. For my 
*In my former discussion, p. 81,1 have given some notes on the methods of 
procedure in this experiment. Tyndall’s paper is printed in Phil. Mag., Vol. 12, 
1856, p. 37. The horizontal friction between the wax and the rigid surfaces 
pressing upon it, when combined with the direct pressure, gives an inclined 
resultant and strains which are rotational excepting along the central vertical line of 
the wax cake. 
