Recent studies of rhyolitic and pyroclastic materials, and in particular of welded tuffs and bentonites, show that they occur over wide areas and in volumes which greatly exceed earlier evaluations. Volcanic ash and bentonite occur in the eastern United States where such materials were long unrecognized. In most of the western States, only preliminary studies have been made, but they indicate the presence of these materials in volumes measured in thousands of cubic miles. Less is known about pyroclastic materials in most other parts of the world, but local studies in New Zealand, Australia, and the Dutch East Indies show immense volumes of pyroclastic materials.

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