What is (a) A soft or porous stone formed by depositions from water, usually calcareous; -- called also calcareous tufa. (b) A friable volcanic rock or conglomerate, formed of consolidated cinders, or scoria called?

tufa — (a) A soft or porous stone formed by depositions from water, usually calcareous; -- called also calcareous tufa. (b) A friable volcanic rock or conglomerate, formed of consolidated cinders, or scoria. (Webster's 1913.)

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