  A wide variety of resistive and field dependent behaviors have been
previously observed in both doped and non-doped Highly Oriented Pyrolytic
Graphite (HOPG). We find HOPG samples to vary significantly in their
temperature dependent resistances, even between portions taken from the same
sample, yet they exhibit consistent non-saturating magnetoresistance (MR). The
scaling behavior of the MR is shown to be characteristic of a model based on
the Hall effect in granular materials. In addition to the large, field-linear
MR, all samples exhibit Shubnikov-de Haas (SdH) oscillations. Additional
samples were doped via ion-implantation by boron and phosphorous, but show no
signs of superconductivity nor any systematic change in their magnetoresistive
behavior. Analysis of the SdH data gives a 2D carrier density in agreement with
previous results, and a large mean-free path relative to crystallite size, even
in samples with thin ion-implanted surface layers.
