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We report the first uniform and systematic study of dust and molecular gas innearby molecular clouds. We use surveys of dust extinction and emission todetermine the opacity and map the distribution of the dust within a dozen localclouds in order to derive a uniform set of basic cloud properties. We find: 1)the average dust opacity $\langle\kappa_{d,353}\rangle = 0.8\ {\rm cm^{2}\,g^{-1}}$ with variations of a factor of $\sim$ 2 between clouds, 2) cloud PDFsare exquisitely described by steeply falling power-laws with a narrow range ofslope, and 3) a tight $M_{\rm GMC} \sim R_{\rm GMC}^2$ scaling relation for thecloud sample, indicative of a cloud population with an exactingly constantaverage surface density above a common fixed boundary. We compare these resultsto uniformly analyzed CO surveys. We measure the CO mass conversion factors andassess the efficacy of CO for tracing the physical properties of molecularclouds. We find $\langle \alpha_{\rm CO}\rangle = 4.31 \pm 0.67$ M$_\odot$ (Kkm s$^{-1}$ pc$^2$)$^{-1}$ (corresponding to $X_{\rm CO}$ = 1.97 $\times$10$^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$). We demonstrate that CO observationsare a poor tracer of column density and structure on sub-cloud spatial scales.On cloud scales, CO observations can provide measurements consistent with thoseof the dust, provided data are analyzed in a similar, self-consistent fashion.Measurements of average GMC surface density are sensitive to choice of cloudboundary. Care must be exercised to adopt common fixed boundaries whencomparing surface densities for cloud populations within and between galaxies.

Systematic Investigation of Dust and Gaseous CO in 12 Nearby Molecular Clouds