240 FRAGMENTS OF SCIENCE. 
cient to account for the cleavage? A single geologist, as 
far as I am aware, answers boldly in the affirmative. This 
geologist is Sorby, who has attacked the question in the 
true spirit of a physical investigator. Call to mind the 
cleavage of the flags of Halifax and Over Darvven, which is 
caused by the interposition of layers of mica between the 
gritty strata. Mr. Sorby finds plates of rnica to be also a 
constituent of slate-rock. He asks himself, what will be 
the effect of pressure upon a mass containing such plates 
confusedly mixed up in it? It will be, he argues, and he 
argues rightly, to place the plates with their flat surfaces 
more or less perpendicular to the direction in which the 
pressure is exerted. He takes scales of the oxide of iron, 
mixes them with a fine powder, and on squeezing the mass 
finds that the tendency of the scales is to set themselves at 
right angles to the line of pressure. Along the planes of 
weakness produced by the scales the mass cleaves. 
By tests of a different character from those applied by 
Mr. Sorby, it might be shown how true his conclusion is 
that the effect of pressure on elongated particles, or plates, 
will be such as he describes it. But while the scales must 
be regarded as a true cause, I should not ascribe to them a 
large share in the production of the cleavage. I believe 
that even if the plates of mica were wholly absent, the 
cleavage of slate-rocks would be much the same as it is at 
present. 
Here is a mass of pure white wax; it contains no mica 
particles, no scales of iron, or anything analogous to them. 
Here is the selfsame substance submitted to pressure. I 
would invite the attention of the eminent geologists now 
before me to the structure of this wax. No slate ever ex- 
hibited so clean a cleavage; it splits into laminae of sur- 
passing tenuity, and proves at a single stroke that pressure 
is sufficient to produce cleavage, and that this cleavage is 
independent of intermixed plates or scales. I have pur- 
posely mixed this wax with elongated particles, and am 
unable to say at the present moment that the cleavage is 
sensibly affected by their presence if anything, I should 
say they rather impair its fineness and clearness than 
promote it. 
The finer the slate is the more perfect will be the resem- 
blance of its cleavage to that of the wax. Compare the 
surface of the wax with the surface of this slate from Bor- 
