The mud balls or pisolites of concentric mud envelopes are formed in connection with hot springs. Some of the mud layers are composed of coarser material cemented by mud--small shreds of glass, scattered broken crystals, numerous tiny tubular masses resembling altered pumice, and an abundance of microscopic colloform bodies showing minute concentric ring systems, apparently made of opal. The possibility that some of the colloform structures might represent algae could not be confirmed. Some may represent silicifications of coniferous wood, others may have originated as cavity fillings.

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