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Molecular gas in other galaxies is generally studied by observations of CO emission; a conversion from CO integrated intensity to H₂ column density must be made. Modelling of the emission from an ensemble of molecular clouds shows that these conversion factors are sensitive to temperature, so that molecular gas masses in galaxies with high star formation rates have probably been overestimated. Conversely, models of molecular clouds in low metallicity systems (such as irregular galaxies) demonstrate that the use of CO as a tracer can severely underestimate the molecular gas abundance. The observed properties of dark clouds and high latitude clouds are consistent with clouds in equilibrium with an intercloud pressure of P/k ≈ 10⁴. Detailed comparison of the CO and 170μm emission from the disks of NGC 6946 and M51 shows that the far-infrared flux must arise from dust in molecular clouds, not atomic clouds; this emission may be powered by embedded young stars or by the interstellar radiation field. The interpretation of the ratio of infrared to CO luminosities as a star formation efficiency is of dubious validity. Modelling of the observed CO and far-infrared emission from a sample of galactic nuclei shows that roughly half of the CO flux is produced by very active star-forming clouds with warm CO. The constraints placed on star formation models by abundance gradients in galaxies suggests that radial gradients in star forming efficiency generally exist in galaxies. The actual distribution of molecular gas in galaxies may be closely tied to the radial mass distribution.

In this thesis we use multi-transitional millimeter/submillimeter-wave molecular spectroscopy of CO and CS to determine the state of the molecular gas in the central regions of three starbursts: M82, IC342, and M83. High angular resolution 60 and 100 μm IRAS images provide complementary information about the thermal dust emission in IC342 and M83. Our CO observations reveal the presence of a molecular ring and supernovae driven wind in M82. In IC342 and M83 there is evidence for molecular bars and central rotating cores. The CO and CS line ratio analyses suggest a multicomponenet medium with clouds externally heated by ultraviolet flux from young, massive stars. Excitation temperatures typically range from 20 to 40 K throughout the nuclear regions of the sample galaxies. In M82 the CO and CS optical depths are ∼ 1. Our analysis of ¹²CO indicates that this gas is optically thick toward the centers of IC342 and M83. The molecular gas mass in each galaxy is ∼ 5x10⁷ M(⊙). We derive an average cloud size between 0.1 and 1 pc in the nuclear region of M82 and M83. An average cloud size of 10 pc is found over a comparable region in IC342. From tidal arguments we find that the clouds must have densities greater than 100 to 1000 cm⁻³ to survive. If the clouds are virialized, then the expected individual cloud linewidths are 9, 40, 5 and 27 km/s for M82, IC342, M83 and the Milky Way, respectively. For the clouds to be pressure-bound, inter-cloud pressures > 10x the peak value in the Galactic Center are required. If the magnetic fields are frozen into the gas, an average field strength of 8.5 mG is needed to support the nuclear clouds in each galaxy from collapse. Enhanced IRAS images reveal bright, compact nuclear components in IC342 and M83. HII regions are seen along spiral arms in IC342 and a dusty bar is seen in M83. The similarity between radio continuum maps and the high resolution IRAS maps suggest that infrared emission arises from HII regions. Using an emissivity law of β ∼ 1.5, the derived dust temperatures in the nuclei of IC342 and M83 are essentially the same as the gas excitation temperatures. For this to occur, gas densities of > 10⁴ cm⁻³ are implied. We derive a star-formation efficiency, ∊, of 77, 60, 10 and 2% for M82, M83, IC342, and the Milky Way, respectively. We find evidence that the gas surface density toward the centers of these galaxies is α ∊. We estimate star-formation rates of 16, 6, 2.5, and .06 M(⊙)/yr for M82, M83, IC342 and the Milky Way. The gas depletion timescales are a few million years for M82 and M83 and a few times 10⁷ and 10⁸ years in IC342 and the Milky Way. We find a strong correlation between cloud diameter and star-formation efficiency, with smaller clouds found in galaxies with higher ∊. We conclude these smaller clouds are a by-product and not a causal factor of the starburst phenomenon.

Silica cemented pedogenic horizons known as duripans occur on relict landforms in climates having limited leaching potential. Under the influence of tephra, incipient silica cementation may also occur in late Quaternary soils. The source of silica for cementation in Holocene soils is rapidly weatherable volcanic glass. In response to wetting and drying cycles volcanic glass weathers to form sand-size composite particles composed of silt, clay and redeposited silica. A portion of the hydrolyzed silica is eluviated as monosilicic acid which accumulates as the wetting front evaporates. Illuvial silica augments existing composite particles eventually forming a continuously cemented duripan. In actively forming late Quaternary soils, the cementation process is a complex interaction between illuvial silica, clay and calcium carbonate and the soil matrix. Surface reactions between monosilicic acid and illuvial clay or soil matrix particles provide nucleation sites for polymerization of silica concentrated by evaporation. The resulting opaline silica bonds adjacent soil grains without necessarily plugging intervening pore spaces. In contrast, calcium carbonate preferentially precipitates in large pores and interped voids. Cementation occurs by the plugging of progressively smaller pores with relatively pure calcite rather than by heterogeneous bonding of mineral grains. In illuvial zones containing both silica and calcium carbonate, cementation may occur rapidly because the former holds small soil particles in place while the latter plugs large pores. The mineralogy of silica cement is determined by identification of varying amounts of crystal order using X-ray diffraction. Opal-A is recently polymerized, noncrystalline, highly hydrated silica gel. The more prevalent, partly crystalline opal-CT forms where surface reaction with clays create crystal orientation, where silica gel dehydrates or when silica precipitates from soil solutions having high concentrations.

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