CHAPTER IV.
PHYSICAL PROPERTIES.
B—Cleavage.
By cleavage
is meant the manner in which minerals separate or split off with
regularity. The difference between a break or fracture and a " cleave,"
is that the former may be anywhere throughout the substance of the
broken body, with an extremely remote chance of another fracture being identical in form, whereas in the latter,
when a body is " cleaved," the fractured part is more readily severed,
and usually takes a similar if not an actually identical form in the
divided surface of each piece severed. Thus we find a piece of wood may
be " broken " or " chopped " when fractured across the grain, no two fractured edges being alike ; but, strictly speaking,
we only " cleave " wood when we " split" it with the grain, or, in
scientific language, along the line of cleavage, and then we find many
j>ieces with their divided surfaces identical. So that when wood is
" broken," or " chopped," we obtain pieces of any width or thickness,
with no manner of regularity of fracture, but when " cleaved," we obtain strips which are often perfectly parallel, that is, of equal thickness throughout their whole length, and of such uniformity of surface that it is difficult or even impossible to distinguish one
