The Collaborative International Dictionary of English v.0.48:

Basal \Ba"sal\, a.
   Relating to, or forming, the base.
   [1913 Webster]

   Basal cleavage. See under Cleavage.

   Basal plane (Crystallog.), a plane parallel to the lateral
      or horizontal axis.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cleavage \Cleav"age\, n.
   1. The act of cleaving or splitting.
      [1913 Webster]

   2. (Crystallog.) The quality possessed by many crystallized
      substances of splitting readily in one or more definite
      directions, in which the cohesive attraction is a minimum,
      affording more or less smooth surfaces; the direction of
      the dividing plane; a fragment obtained by cleaving, as of
      a diamond. See Parting.
      [1913 Webster]

   3. (Geol.) Division into lamin[ae], like slate, with the
      lamination not necessarily parallel to the plane of
      deposition; -- usually produced by pressure.
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   Basal cleavage, cleavage parallel to the base of a crystal,
      or to the plane of the lateral axes.

   Cell cleavage (Biol.), multiplication of cells by fission.
      See Segmentation.

   Cubic cleavage, cleavage parallel to the faces of a cube.


   Diagonal cleavage, cleavage parallel to ta diagonal plane.


   Egg clavage. (Biol.) See Segmentation.

   Lateral cleavage, cleavage parallel to the lateral planes.


   Octahedral cleavage, Dodecahedral cleavage, or
   Rhombohedral cleavage, cleavage parallel to the faces of an
      octahedron, dodecahedron, or rhombohedron.

   Prismatic cleavage, cleavage parallel to a vertical prism.
      [1913 Webster]