  Strain in granular materials in quasistatic conditions under varying stress
originate in (I) contact deformation and (II) rearrangements of the contact
network. Depending on sample history and applied load, either mechanism might
dominate. One may thus define rheological regimes I and II accordingly. Their
properties are presented and illustrated here with discrete numerical
simulation results on sphere packings. Understanding the microscopic physical
origin of strain enables one to clarify such issues as the existence of
macroscopic elasticity, the approach to stress-strain relations in the large
system limit and the sensitivity to noise.
