Regionally extensive dacite-andesite and rhyodacite porphyries within the mid-Paleozoic flyschlike fill of the Hill End Trough of New South Wales have characteristics that are consistent with their being lava flows: conformable, no crosscutting relationships; planar coherent, irregular autobrecciated and quenched margins; vesicular; internally coherent and not vitriclastic; internally uniformly porphyritic and not zoned; and observable topographic influence on the character of succeeding stratigraphic units.

The relatively deep marine environment that is inferred from the flyschlike nature of the enveloping stratigraphic succession may be responsible for the regionally extensive character of these lavas. Although subaerial equivalents are demonstrably viscous and immobile, subaqueously erupted and emplaced silicic lavas may behave fluidly and may be highly mobile because of the inability of volatiles to escape under conditions of high environmental (water) pressures.

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