Recent high resolution surveys with the ISO and MSX satellites have revealed a large number of Galactic clouds with significant extinction in the mid-infrared. The infrared (IR) dark clouds are characterized by high masses, high column densities, and low temperatures. Little is known, however, about their origin and distribution in the Galaxy. The BU-FCRAO Galactic Ring Survey (GRS), a high resolution survey of 13CO emission in the inner Milky Way, makes it possible to spectroscopically determine their distances, and hence sizes and masses, based on morphological correlation of IR extinction and GRS 13CO emission. We find that the majority of the IR dark clouds are concentrated in the Galactic Ring at a Galactocentric radius of 5 kpc. We suggest that the condensations in the most massive IR dark clouds represent high mass proto-clusters, or OB-associations in the making.
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