  (Abridged) We use 8 micron Spitzer GLIMPSE images to make extinction maps of
10 IRDCs, selected to be relatively nearby and massive. The extinction mapping
technique requires modeling the IR background intensity behind the cloud, which
is achieved by correcting for foreground emission and then interpolating from
the surrounding regions. The correction for foreground emission can be quite
large, thus restricting the utility of this technique to relatively nearby
clouds. We investigate three methods for the interpolation, finding systematic
differences at about the 10% level, which, for fiducial dust models,
corresponds to a mass surface density Sigma = 0.013 g cm^-2, above which we
conclude this extinction mapping technique attains validity. We examine the
probability distribution function of Sigma in IRDCs. From a qualitative
comparison with numerical simulations of astrophysical turbulence, many clouds
appear to have relatively narrow distributions suggesting relatively low (<5)
Mach numbers and/or dynamically strong magnetic fields. Given cloud kinematic
distances, we derive cloud masses. Rathborne, Jackson & Simon identified cores
within the clouds and measured their masses via mm dust emission. For 43 cores,
we compare these mass estimates with those derived from our extinction mapping,
finding good agreement: typically factors of <~2 difference for individual
cores and an average systematic offset of <~10% for the adopted fiducial
assumptions of each method. We find tentative evidence for a systematic
variation of these mass ratios as a function of core density, which is
consistent with models of ice mantle formation on dust grains and subsequent
grain growth by coagulation, and/or with a temperature decrease in the densest
cores.
